feat: switch from tsup to tsdown (#2224)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #2223
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Switches over, and includes intake/transition logic to remove the
`bundle` property from old `tsup.config.ts` files.

~~Blocked from passing lint:spelling on
streetsidesoftware/cspell-dicts#4518 -> #2237.~~
✅

🎁

Author: JoshuaKGoldberg

.github/DEVELOPMENT.md

@@ -16,7 +16,7 @@ pnpm install
 
 ## Building
 
-Run [**tsup**](https://tsup.egoist.dev) locally to build source files from `src/` into output files in `lib/`:
+Run [**tsdown**](https://tsdown.dev) locally to build source files from `src/` into output files in `lib/`:
 
 ```shell
 pnpm build

docs/Blocks.md

@@ -50,16 +50,16 @@ This table summarizes each block and which base levels they're included in:
 | Renovate                     | `--add-renovate`, `--exclude-renovate`                                         |         |        | 💯         |
 | Security Docs                | `--add-security-docs`, `--exclude-security-docs`                               | ✔️      | ✅     | 💯         |
 | Templated With               | `--add-templated-with`, `--exclude-templated-with`                             | ✔️      | ✅     | 💯         |
-| TSup                         | `--add-tsup`, `--exclude-tsup`                                                 | ✔️      | ✅     | 💯         |
+| TSDown                       | `--add-tsdown`, `--exclude-tsdown`                                             | ✔️      | ✅     | 💯         |
 | TypeScript                   | `--add-typescript`, `--exclude-typescript`                                     | ✔️      | ✅     | 💯         |
 | Vitest                       | `--add-vitest`, `--exclude-vitest`                                             |         | ✅     | 💯         |
 | VS Code                      | `--add-vs-code`, `--exclude-vs-code`                                           |         |        | 💯         |
 | Web-ext                      | `--add-web-ext`, `--exclude-web-ext`                                           |         |        |            |
 
-For example, this uses ncc instead of the default tsup builder:
+For example, this uses ncc instead of the default tsdown builder:
 
 ```shell
-npx create-typescript-app --add-ncc --exclude-tsup
+npx create-typescript-app --add-ncc --exclude-tsdown
 ```
 
 See also [CLI](./CLI.md) for customizing templated repositories when running `npx create-typescript-app`.
@@ -82,7 +82,7 @@ We strongly recommend using at least the [_"common"_ base level](#common-base-le
 
 ### Building
 
-[**tsup**](https://tsup.egoist.dev): Builds output definitions and JavaScript files using [esbuild](https://esbuild.github.io).
+[**tsdown**](https://tsdown.dev): Builds output definitions and JavaScript files.
 Each `*.ts` source file within `src/` is built into `.d.ts` and `.js` output files in `lib/`.
 
 Building once:

docs/FAQs.md

@@ -66,17 +66,17 @@ Here we'll outline the steps required to migrate a CTA app to a GitHub Action:
       </details>
 
    - GitHub Actions run without installing package dependencies.
-     Replace `tsup` with [`ncc`](https://github.com/vercel/ncc) to build source files and dependencies into a single JS file.
-     Delete `tsup.config.ts` then execute the following commands:
+     Replace `tsdown` with [`ncc`](https://github.com/vercel/ncc) to build source files and dependencies into a single JS file.
+     Delete `tsdown.config.ts` then execute the following commands:
 
    ```bash
-   pnpm remove tsup
+   pnpm remove tsdown
    pnpm add @vercel/ncc -D
    ```
    - Now we need to update the `build` script in our `package.json`:
 
    ```diff
-   -"build": "tsup",
+   -"build": "tsdown",
    +"build": "ncc build src/index.ts -o dist --license licenses.txt",
    ```
    - Our build now emits to the `dist` directory; so we'll want to avoid linting that directory by adding the following to `.eslintignore` and our `.prettierignore`:
@@ -102,7 +102,7 @@ First, I'd suggest reading [TypeScript Handbook > Modules - Introduction](https:
 
 Then:
 
-1. In `tsup.config.ts`, change the [tsup `format` option](https://tsup.egoist.dev/#bundle-formats) from `["esm"]` to `["cjs", "esm"]`
+1. In `tsdown.config.ts`, change the [tsdown `format` option](https://tsdown.dev/options/output-format) from `["esm"]` to `["cjs", "esm"]`
 2. Add a [`package.json` `"exports"` entry](https://nodejs.org/api/packages.html#subpath-exports) like:
 
    <!-- eslint-disable jsonc/sort-keys -->

package.json

@@ -22,7 +22,7 @@
 		"package.json"
 	],
 	"scripts": {
-		"build": "tsup",
+		"build": "tsdown",
 		"format": "prettier .",
 		"lint": "eslint . --max-warnings 0",
 		"lint:knip": "knip",
@@ -113,7 +113,7 @@
 		"prettier-plugin-sh": "0.17.4",
 		"release-it": "19.0.3",
 		"sentences-per-line": "0.3.0",
-		"tsup": "8.5.0",
+		"tsdown": "0.12.7",
 		"typescript": "5.8.3",
 		"typescript-eslint": "8.35.0",
 		"vitest": "3.2.4"