Treat ASCII/non-ASCII transitions as word boundaries in text area

[kyu08]

PR Description

Resolves #5805

Treat ASCII/non-ASCII transitions as word boundaries in text area in these operations:

1. Word deletion(textarea.BackSpaceWord()/textarea.ForwardDeleteWord())
2. Word movement(textarea.MoveRightWord()/textarea.MoveLeftWord())

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc