Support default value in template?

[kamontat]

Description

I would love of we could define default value when template variable is not available.

What problem it is solving?

My use-case is I would like to have share config like

commit-msg:
  commands:
    check:msg:
      run: {check:msg:cmd?pnpm} check:msg --edit {1}

then on project config, we could do like

## Use default pnpm command
extends:
  - ../shared/lefthook.yaml
---

## Override command to npm instead
extends:
  - ../shared/lefthook.yaml
templates:
  "check:msg:cmd": npm