Introduce prettier to format JavaScript (.js) and Sass (.scss) files

[TymekDev]

• Mentioned on Slack (internal)
• Unblocks Upgrade stylelint #521

Description

With this PR I introduce two new functions: format_js() and format_sass(). Both of these take a fix argument that defaults to TRUE to allow just checking the style.

Important

There might be places where prettier's formatting might be incompatible with AirBnB and/or stylelint linters, but I am not aware of these.

How to Test

A New App

1. Install Rhino from the branch
2. Create a new app with rhino::init()
3. Add some unformatted code in app/js/ and app/styles/ directories
4. Run format_js() and format_sass() to see if they work

An Existing App

1. Run renv::install("TymekDev/rhino@formatters")
2. Remove .rhino/ directory
3. Make sure there is unformatted code in app/js/ and app/styles/ directories
4. Run format_js() and format_sass() to see if they work

Definition of Done

• The change is thoroughly documented.
• The CI passes (R CMD check, linter, unit tests, spelling).
• Any generated files have been updated (e.g. .Rd files with roxygen2::roxygenise())

[jakubnowicki]

LGTM, please address my comments and you are ready to go.

[jakubnowicki]

Please bum the package version.

[jakubnowicki]

Please add an entry for the migration guide (remove .rhino etc.)

[kamilzyla]

Merged as #597.