Partially addresses #25698

Part of why autocomplete suggestions for `keymap.json` aren't great is
because `:` is (correctly) considered a punctuation character, rather
than a word character, in JSON. But since `::` is part of the name of
zed commands, it means that the autocomplete context window loses
context after the user types colon:

Suggestion here is to use overrides for JSON and JSONC such that colon
is considered a word character when it's inside a string. This improves
the experience:

I believe this is more broadly correct anyway, since `:` loses it's
punctuation meaning when inside a string.

Hope this is helpful!

Release Notes:

- Improved autocomplete for keymap.json by treating `::` like word characters when inside a string.