Implement code formatting

#38 is a discussion of possible approaches. This issue tracks implementation progress for the current approach.

• add CLI subcommand with options and file processing https://github.com/gren-lang/compiler/pull/46
  • format directories implied by gren.json https://github.com/gren-lang/compiler/pull/46
  • --yes https://github.com/gren-lang/compiler/pull/46
  • --validate https://github.com/gren-lang/compiler/pull/143
  • explicit INPUTS... https://github.com/gren-lang/compiler/pull/46
  • --stdin https://github.com/gren-lang/compiler/pull/46
• modify parser/AST to retain comments and necessary whitespace info
  • modify whitespace parser to parse comments https://github.com/gren-lang/compiler/pull/56
  • https://github.com/gren-lang/compiler/pull/132
  • https://github.com/gren-lang/compiler/pull/142
  • https://github.com/gren-lang/compiler/pull/153
  • https://github.com/gren-lang/compiler/pull/154
  • https://github.com/gren-lang/compiler/pull/159
  • https://github.com/gren-lang/compiler/pull/160
  • https://github.com/gren-lang/compiler/pull/161
  • https://github.com/gren-lang/compiler/pull/162
  • https://github.com/gren-lang/compiler/pull/165
  • TBD: retain comments part N
  • retain comments in empty arrays and records
• implement the formatting logic
  • initial implementation (everything no requiring whitespace info should work) https://github.com/gren-lang/compiler/pull/117
  • allow kernel module features when formatting kernel modules
  • TBD any adjustments to formatting rules
    • comments at https://github.com/gren-lang/example-projects/pull/11
  • TBD fix anything that's broken
    • https://github.com/gren-lang/compiler/pull/164
• add formatting test suite
  • basic pattern established in https://github.com/gren-lang/compiler/pull/132
  • (maybe) a property test to ensure that formatting is idempotent
• cleanup
  • extract Text.PrettyPrint.Avh4 to a separate library to share with elm-format

Planned CLI options:

gren format [INPUTS... | --stdin] [--yes] [--validate]

• INPUTS: zero or more files to format, or directories to recursively search for gren files to format. If none are given, use the directories implied by gren.json
• --stdin: instead of formatting files, read stdin and format it to stdout
• --yes: don't prompt for confirmation before formatting files and overwriting them (does nothing with --stdin)
• --validate: Don't modify files, and exit 0 if files are all formatted, or exit 1 if some files are not formatted. Intended for use on CI.

Start with the unchecked parser/AST comment-retention items and the formatting test suite described in the issue. Review the planned gren format CLI options and the existing linked implementation work before proceeding. Done means completing the remaining formatting, test, and cleanup checklist items, including idempotence if retained.