[pigeon] Improve style of generated Swift code

[stuartmorgan-g]

Cleans up our Swift output to be more idiomatic (or in a couple of cases to match swift-format` in particular):

• Don't indent cases in switches.
• Remove some stray ;s
• Remove ()s around if conditions
• Add trailing newlines in multi-element arrays
• Fix some missing standard whitespace in a handful of places where it was missing (after named arguments, between () and {)
• Indent inside #if constructs.
• Two changes that are more arguable, as it's not clear that they are non-idiomatic vs just non-swift-format-style (and they add branching to the generator that would otherwise not be needed):
  • Don't put a trailing comma on the list element of a one-element list
  • Don't put () on zero-arg method calls with a trailing closure

With these changes, as far as can tell the diffs created by swift-format are just line wrapping.

Fixes flutter/flutter#141799

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

Let me know if you want me to revert the two special-case style things at the end of the list. I went back and forth on whether to write them, and still don't feel strongly either way.

[tarrinneal]

Let me know if you want me to revert the two special-case style things at the end of the list. I went back and forth on whether to write them, and still don't feel strongly either way.

I actually would prefer to keep those implementations as well.

[tarrinneal]

These look good to me, thanks @stuartmorgan

[auto-submit]

auto label is removed for flutter/packages/5938, due to - The status or check suite Linux_android custom_package_tests stable has failed. Please fix the issues identified (or deflake) before re-applying this label.