Ref T13395. Merge a lot of stuff which doesn't break existing workflows:
- Merge a UTF8 fix for "cmd.exe" on Windows.
- Merge minor changes to JSON linters.
- Merge some shell completion stuff.
- Merge some "arc anoid" fixes.
- Merge various Windows improvements to unit tests which interact with processes / the filesystem.
- Merge some other Windows path fixes.
- Merge a UTF8 character class fix.
- Merge script initialization.
- Merge unit test support scripts.
- Merge some initialization code.
- Merge Windows stdout/stderr-as-files code.
- Merge a bunch of code for making exec tests work on Windows.
- Merge more Windows unit test fixes.
- Merge "continue on failure" mode when loading symbols.
- Merge "GPC" order CLI fixes.