[Feature request] Case-insensitive string comparison operator

We have `===` and `!==`, which are terser than `STRCMP`, but there's still a common idiom of doing case-insensitive comparisons, usually for instruction-like macros (e.g. `strlwr("\1") === "hl"` or `strupr("\1") === "HL"`, doesn't matter which).

It could be `~==`, e.g. `"\1" ~== "hl"`. (And don't bother with a "compare case-insensitively different" operator, something like "`!~=`" or "`~!=`" or even "`!~==`" would be ugly and too rare to bother.)

The issue proposes a new case-insensitive string comparison operator, using examples with strlwr, strupr, ===, !==, and STRCMP. No implementation files, tests, or entry points are named, so first locate the existing comparison-operator handling and its tests; done should include an agreed operator syntax and coverage for case-insensitive comparisons.

Written by the indexing model from the issue text.