build: Add CI and Task for formatting R and Rust code

[etiennebacher]

Fixes #72

Putting this in draft until a few issues and features are complete in air, specifically:

• ignore files generated by savvy: Add a default set of files / folders to ignore posit-dev/air#128 (comment)
• formatting for calls to tribble(): Tabular formatting posit-dev/air#113. Currently those are completely expanded vertically, e.g.:

tibble::tribble(
  ~.test_name, ~object,
  "Int8", pl$Int8,
  "Int16", pl$Int16,
  "Int32", pl$Int32,
  "Int64", pl$Int64,
  "UInt8", pl$UInt8,
)

becomes:

tibble::tribble(
  ~.test_name,
  ~object,
  "Int8",
  pl$Int8,
  "Int16",
  pl$Int16,
  "Int32",
  pl$Int32,
  "Int64",
  pl$Int64,
  "UInt8",
  pl$UInt8,
)

---

When those are fixed, I'll make a separate PR where I actually run the formatters so that this PR is not drowned in formatting changes.

[etiennebacher]

This is downloaded every time task fmt runs, which is bad. @eitsupi do you know why this is executed while other setup tasks are skipped? Is it because this is a curl command and there's no way to know if it's outdated?