Adds a node-compatible export

[gkjohnson]

Fix #9

• Moves the node variant of the unwrapper to a node.ts file.
• Adds a separate node entry to the webpack build so two entry points are built (one for browser, one for node)
• Adds a second import path xatlas-three/node so the node variant can be used easily.
• Automatically finds the xatlasjs files and initializes the wasm paths so there's no need to call "loadLibrary" explicitly. Unfortunately I don't think this can be done for the browser variant easily due to the state of workers and wasm imports...
• Add "xatlasjs" dependency since it's required for node support, now.
• Await the library to be loaded in the base unwrapper

Once node is working correctly there are a few things I'd like to adjust in future PRs if that's okay - let me know how this sounds:

• Add a "verbose" flag and / or log callback to enable / disable the automatic logging that occurs when loading the xatlasjs files.
• Add a "dispose" function to dispose of the wasm memory and workers used by the unwrappers.
• Remove the need to explicitly pass in three.js' BufferAttribute to the constructor.

[gkjohnson]

Typescript seems to not like this spread syntax for passing through the arguments to the parent constructor. I looked it up but there didn't seem to be a clear solution. Do we have to reproduce all the arguments and types for this constructor?

[gkjohnson]

I'm also seeing this warning when running the node instance - looks like there's maybe a memory leak in xatlasjs? Or "comlink"?

(node:91465) MaxListenersExceededWarning: Possible EventTarget memory leak detected. 11 message listeners added to [MessagePort [EventTarget]]. Use events.setMaxListeners() to increase limit
    at [kNewListener] (node:internal/event_target:534:17)
    at MessagePort.<anonymous> (node:internal/worker/io:308:12)
    at MessagePort.addEventListener (node:internal/event_target:645:23)
    at /Users/garrett/Desktop/git/xatlas-three/node_modules/comlink/dist/umd/comlink.js:331:16
    at new Promise (<anonymous>)
    at requestResponseMessage (/Users/garrett/Desktop/git/xatlas-three/node_modules/comlink/dist/umd/comlink.js:329:16)
    at Object.apply (/Users/garrett/Desktop/git/xatlas-three/node_modules/comlink/dist/umd/comlink.js:257:24)
    at _onAtlasProgress (/Users/garrett/Desktop/git/xatlas-three/node_modules/xatlasjs/dist/node/xatlas.js:9:63204)
    at wasm://wasm/0008d70a:wasm-function[31]:0xbf8
    at wasm://wasm/0008d70a:wasm-function[133]:0xdfc7