buildifier: improve Windows runner performance (#1404)

* buildifier: improve Windows runner performance

This change aims to improve the `buildidier`'s runner performance on
Windows where it can sometimes take minutes to complete.

It consists in replacing the slow COM `Scripting.FileSystemObject` with
a "native" PowerShell `Get-ChildItem -Recurse` for file discovery, and
batch `buildifier` invocations (100 files at a time) instead of invoking
once per file.

This improves performance on large codebases (like from about 2 minutes
to less than 3 seconds).

Notes:
- cross-process COM calls add latency, whereas `Get-ChildItem` is a
  compiled "cmdlet".
- by default, `Get-ChildItem` doesn't recurse into symbolic links to
  directories, which is consistent with the current implementation that
  explicitly avoids entering `ReparsePoint`s,
- batching files passed to `buildifier` reduces process creation
  overhead, with a limit to account for Windows command line length
  limitations.

References:
- https://stackoverflow.com/questions/73893997/comments-in-a-long-line-powershell-code-in-a-batch-script
- https://stackoverflow.com/questions/3205027/maximum-length-of-command-line-string
- https://devblogs.microsoft.com/oldnewthing/20031210-00/?p=41553