emacs: Attributes highlighting fixes

[tomjakubowski]

This addresses two problems noted in #14347: the highlight of #foo] as an attribute, and the non-highlight of #[foo = "bar"].

[tomjakubowski]

Here's an example screenshot:

[emberian]

cc @brson @nikomatsakis @pnkfelix, surely there is an emacser who can review this.

[pnkfelix]

@tomjakubowski I tried to find documentation on what the trailing ... t) that you added signifies in a font-lock-defaults list. But alas I was not able to find a description of it. Would you mind pointing me at some docs for it?

(Having said that, I will r+ this now, since I trust that it is an improvement on what we had before, especially with the bug fix to the regexp.)

[tomjakubowski]

@pnkfelix the relevant documentation is in M-x describe-variable font-lock-keywords.

MATCH-HIGHLIGHT should be of the form:

 (SUBEXP FACENAME [OVERRIDE [LAXMATCH]])

...

OVERRIDE and LAXMATCH are flags.  If OVERRIDE is t, existing fontification can
be overwritten.  If `keep', only parts not already fontified are highlighted.
If `prepend' or `append', existing fontification is merged with the new, in
which the new or existing fontification, respectively, takes precedence.

The problem is that Emacs's highlighting for matched delimiters in the syntax table take precedence over keywords. Thinking it over some more we should be using append; t makes Emacs over-eagerly highlight attributes inside comments and string literals:

I'll submit another PR with that fix.