Fix nondeterministic highlighting

[A-Walrus]

Resolves #2986.
Resolves #3230.
Probably resolves #2484 as well, although I didn't have the code to copy paste in order to check :P

This is done by preferring matches in the beginning, i.e. for keyword.function, keyword is a better match than function.

Explanation of original problem:

Source for non-determinism: HashMap traverse order is different each time.

According to comments:

Consumers of these queries can choose to recognize highlights with different levels of specificity.
For example, the string function.builtin will match against function.method.builtin
and function.builtin.constructor, but will not match function.method.

keyword.function can therefore match both keyword and function.

• Both of these matches have the same match_len which is what is used to rank them
• The list of scopes is traversed in order to find the match with the best match_len, in the
  random order it came from the HashMap.

[archseer]

There's an easier fix here: I think the original intends for matches to be from the start of the string. So keyword.function matches keyword, keyword.function, but not function.

[the-mikedavis]

Nice, thanks for the fix!

I think the old behavior may have been intentional but I think the new way is more straightforward and intuitive (plus less edge-case-y)

And thanks for updating the docs too :)

[archseer]

Thanks for fixing this! I'm really glad this was resolved 🎉