AK: Store JSON keys and values as WTF-8 strings

[yyny]

JSON can only contain WTF-8 encoded text, so it makes sense to type them as such. Also turn JSON keys into FlyStrings, to reduce memory usage and increase performance of lookups.

Part of the improvements made in #2493.

[AtkinsSJ]

This has conflicts.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[github-actions[bot]]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!