[8.1.0] Add `--incompatible_enforce_starlark_utf8` (#25152)

If enabled (or set to `error`), fail if Starlark files are not UTF-8
encoded. If set to `warning` (the default), emits a warning instead.

Bazel already assumes that Starlark files are UTF-8 encoded for e.g.
filenames in actions executed remotely. This flag doesn't affect this,
it only makes encoding failures more visible.

Work towards #374

Closes #24944.

PiperOrigin-RevId: 721513249
Change-Id: I1d3363168c6cd5d37abf96e0401e34866b6679d7

(cherry picked from commit e7934ce9)

Fixes #25148