[8.1.0] Add --incompatible_enforce_starlark_utf8

[iancha1992]

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

Commit e7934ce

[iancha1992]

@tetromino could you please take a look at the presubmit errors? Thanks

[fmeum]

@iancha1992 I sent #25152. The diff, in case you would rather apply it here:

diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java
index 74b975ac71..22e4b73e4d 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java
@@ -439,7 +439,7 @@ public class WorkspaceFileFunction implements SkyFunction {
         StarlarkUtil.InvalidUtf8Exception e) {
       throw resolvedValueError("Failed to read WORKSPACE file");
     }
-    StarlarkFile file = StarlarkFile.parse(parserInput);
+    StarlarkFile file = StarlarkFile.parse(parserInput, options);
     if (!file.ok()) {
       Event.replayEventsOn(env.getListener(), file.errors());
       throw resolvedValueError("Failed to parse WORKSPACE file");