Add config option for case-insensitive search

[codegax]

Adds a case_insensitive_search setting (default off) so searching rooms, DMs, spaces, members, and message scrollback ignores case without needing a (?i) prefix.

[VAWVAW]

In my opinion it would be more consistent to use the vim naming of igorecase for the config option.

And I believe it makes more sense to implement most of this functionality in upstream modalkit-ratatui by adding something like set_ignorecase() to ListState.

[codegax]

Yeah, good points. Renamed it to ignorecase so it matches Vim.

I'll put together a PR on modalkit-ratatui to add ignorecase to ListState, then switch this over to use it. Only catch is the scrollback search (the / one inside a room) builds its regex in iamb rather than modalkit, so I'll probably still need a little something here for that bit.

[VAWVAW]

In case you want to keep contributing, I want to warn you that this is a really slow moving project and that it can take a few months until changes are merged.

I'll also point you to the matrix channels #iamb-users:0x.badd.cafe and #iamb-dev:0x.badd.cafe and my fork where I merge most all recent PRs together.