CQL: CASEI / ACCENTI support

[m-mohr]

In addition to LIKE we should also give the user an option to compare case-insensitive with ILIKE.

I need to check whether that's a separate conformance class or whether it's always present.

[m-mohr]

There's no ILIKE in OGC API - Features: Part 3 / CQL. It seemt to be handled via CASEI and ACCENTI, which is much more difficult to implement in STAC Browser.

For ILIKE there's a closed PR: #311

See:

• https://docs.ogc.org/DRAFTS/21-065.html#case-insensitive-comparison
• https://docs.ogc.org/DRAFTS/21-065.html#accent-insensitive-comparison