Add support for case-sensitive queries

[rylwin]

Add _ZO_CASE_SENSITIVITY env var to control whether queries should be performed in a case-sensitive manner, keeping the current behavior (case-insensitive) as the default.

Setting _ZO_CASE_SENSITIVITY=case-sensitive changes the query behavior to be case-sensitive.

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I switched to zoxide a few months ago and have found it to be a much more robust tool than what I was using before. I also enjoy all of the integrations it has.

One thing I miss from my previous jump tool is case-sensitivity: I have some dirs that happen to have capitalization that I can use to get to exactly where I want to go, with a single character, as long as the search is case sensitive.

This PR adds case-sensitivity as an option. This could also lay the groundwork for a "smart case" variant (requested in #224)

Also, I've done minimal rust, so if anything here is not idiomatic / should be adjusted happy to make any changes.

[rylwin]

I've rebased this branch on top of latest main

[azaleacolburn]

This looks good to me!

[andy5995]

I'd be very interested in this change. I've been using zoxide for about 6 weeks now (I heard about it on the Linux Cast youtube channel), and case insensitivity is the only thing I can think that I'd want to change. A very handy utility, thank you.