[command-parser] Add config to have commands be case-insesitive

[twilight-rs]

[command-parser] Add config to have commands be case-insesitive

[Erk-]

I have thought about using something like https://crates.io/crates/unicase which would handle case insensitivity

[HiruNya]

Would it only have to be case insensitive for ASCII characters?
I ask because str::eq_ignore_ascii_case only works for ASCII characters and the other possible ways I can think of implementing this require extra String allocations due to the possibility of Unicode characters changing length.

[HiruNya]

Ah yes, that should work. Thanks!

[Erk-]

Actually it may be harder than that, at least with the way the parser is currently structured. As you can not use UniCase in .start_with which may be an issue if we supports non separated commands.
Non seperated commands also gives the other issue that it may not always give the same result, but I will have to look a bit more into that and open a new issue.

[HiruNya]

Yeah I hit that part last night.
I was able to make it work on the assumption that the length would be the same so that a command like !weiß would able to match on !WEISS as ß and SS are both two bytes. However characters like ff wont match on FF.
I played around with a few of these characters but I can't really figure out a way to make them work.
For future reference, the Unicode website has a text file called SpecialCasing.txt with characters that go from one to many characters when upper cased.

[Erk-]

Also with that assumption you are going to run into problems with Greek small sigma, as you don't know if is the end of the word, but if we go with the fix in #81 for #80 it should be possible to do with unicase.

[HiruNya]

Sigma is one of the cases where the assumption would actually work as all three variants are 2 bytes long.

Regarding fix #81, wouldn't using trim_start_matches allow the command to be repeated (e.g. the command "echo" would match "echoecho") and in that case would unicase work?