ignore case while filtering completions

[pascalkuthe]

Fixes #2017

This PR changes the fuzzy matching in the completion menu to be always case insensitive.
We never really documented that it used smart casing and I don't think that is actually desirable here.
A common use-case for competitions is mistyping/misremembering an identifier and using autocomplete to fixup that problem. For that use-case case sensitive matching is actually detrimental. Other editors like nvim/vscode/intellij also seem to always use case insensitive auto completions.

Note that the original reproduction case from #2017 already seems to work:

However after this fix the following also works which doesn't work on master (see screenshot)

Note that we rank completions by their fuzzy matching score which gives a penalty for case mismatch in case ignore case is enabled soo if both Foo and FOO exist both would show up but Foo would be ranked first.