icalvalue.c: validate COLOR values

[ksmurchison]

Per RFC 7986 Section 5.9, and as updated by
draft-ietf-calext-icalendar-jscalendar-extensions Section 2.2:

The value is a color name taken from the set of names defined in Section 4.3 of https://www.w3.org/TR/css-color-3/
or an RGB value in six-digit hexadecimal notation, as defined in Section 4.2.1 of https://www.w3.org/TR/css-color-3/
Values are case-insensitive.

[rsto]

I don't think icalderivedvalue.c belongs in here?

Other than that, this looks sane to me. But: the next step after having fixed up this code is NOT to merge it. Instead:

1. Create a cyrus-imapd branch, update its Github workflow to build and install libical using the branch of this pull request. There should be tests in that cyrus-imapd branch that fail unless libical supports COLOR validation, e.g. they should fail for the current libical cyruslibs tag. The tests should pass with this libical branch. If CI succeeds for this PR, then you know the PR uses the right libical patch and that patch does the right thing (according to the tests).
2. Create a pull request on libical upstream and get it merged.
3. Update the tip of the cyrusimap/libical repo to point to the latest main branch that includes the merged changes. Repeat step 1 with it.

After that, cyruslibs tags will get to get updated and some additional at Fastmail need to happen. We'll do that later.