feat: Format R code with air

[gadenbuie]

Adds a composite workflow rstudio/shiny-workflows/format-r-code that is included into the routine workflow to format R code with air.

This composite workflow can be used anywhere with

- name: Format R code
  uses: rstudio/shiny-workflows/format-r-code@v1

In the routine workflow, I've put the formatting step behind inputs.format-r-code, which is false by default. To turn it on:

  routine:
    uses: rstudio/shiny-workflows/.github/workflows/routine.yaml@feat/format-r-code # @v1
    with:
      format-r-code: true

That lets us opt-in slowly initially or eventually we can change the default value and start formatting everything.

The composite workflow has additional inputs:

• version (default: latest) - which version of air to use so that we could pin to a specific version.
• check (default: false) - when "true" calls air format --check, which exits with a non-zero status if any files change, no changes are committed
• path (default: .) - paths passed to air format ..., can be files or directories, e.g. R inst to format R files in R/ and inst/ only. The current default formats all R files in the repo (aggressive but useful).

I've used this workflow in bslib as a demo: rstudio/bslib#1167

[gadenbuie]

I'm going to merge this, but feel free to leave comments here and I'll follow up.

[schloerke]

Thank you for making it opt-in!