Fix SemanticsAction naming consistency

[cbracken]

Removes the Index suffix from kMoveCursorForwardByWord and
kMoveCursorBackwardByWord values in the SemanticsAction enum. These were
erroneously copied as-is from the private class members in the dart:ui
implementation in lib/ui/semantics/semantics.dart where the class
members refer to the enum index here. This removes the trailing Index
for consistency with the other enum members here.

This is also useful in the context of automated testing for API
consistency between these enums, the ones in dart:ui (native and web
implementations) and the embedder API.

Issue: flutter/flutter#101217

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 Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[cbracken]

No tests since this is just a renaming of a private API. Test will be added in flutter/flutter#101217 which this is pre-factoring for.

Verified offline with @lucasradaelli that the string change is safe for Fuchsia.

[cbracken]

One might ask who was the one who erroneously added this with Index on the end... let's just say this fix has no effect on the author listed on the blame for those lines.