Improve Skeleton3D::find_bone() performance

[44zb]

Indexing Skeleon3D bones using HashMap

results of the script from #75915 before/after this fix
before
Time find_bone(): 1055
Time Dictionary: 59

After
Time find_bone(): 65
Time Dictionary: 57

Fix #75915

[kleonc]

@44zb LGTM besides the PR/commit title. No need for these to link to the issue being fixed (#75915)(it could be done in the PR/commit description). Also more human-readable titles are preferred. So I suggest changing both PR title and commit title (first line of the commit message) to e.g. "Improve Skeleton3D::find_bone() performance".

Also seems like there's some formatting issue, should be addressed too (see the failing test)(Yeah, I suggested it like that, oops! 🙃).

[kleonc]

@44zb Oh, you actually still haven't changed the commit message, it's still "Skeleton3D::find_bone() performance #75915". You can change it when rebasing by using -r/--reword for the given commit.

[44zb]

@kleonc yes, sorry missed the commit part. But now it's all good :)

[fire]

As long as this works for all the input cases and the performance is faster I'm fine with it.

I don't think this portion of the code requires special memory allocation strategies like not allocating.

[clayjohn]

Looks great!

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[YuriSizov]

Cherry-picked for 4.0.4 with #77874.