syntax: Reintroduction of header as fallback when filename doesn't match

[JoeKar]

Fixes #3201

[JoeKar]

@dmaluka:
The MacOS build run one time fail, but currently I can't image why:
https://github.com/zyedidia/micro/actions/runs/8402687727/job/23012403628

I removed my last commit, added it partly again: ok
I removed my last commit, added it fully again: ok

Do you have an idea? I'd like to prevent further regressions.

Strange 🤔

[dmaluka]

I haven't reviewed this PR yet (obviously) but I'm not sure if the extra complexity it adds is the right approach to the problem. The reason is: I've found that #2819, among other things, changed the meaning of the foundDef variable, so now it means not "we found the final syntax definition" but something different, and rather confusing. Which makes the entire code confusing, difficult to understand (and thus difficult to extend in a correct way), and actually buggy.

I already have a fix for foundDef but I haven't fully tested it yet.

After fixing foundDef, adding support for header matches together with signature matches should become trivial (I suppose).

[JoeKar]

Closed as tracked with #3208 due to more clean and straight logic.