Add clojure-discard-face

[Ramblurr]

This allows for font-locking expressions discarded with the #_ form
differently than comments. Maintains default by inheriting from
font-lock-comment-face.

Fixes #688

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• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
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[Ramblurr]

I haven't updated the tests yet.. I'm having trouble getting eldev working on my system. Any tips on how to get or execute the eldev binary available in NixOS would be appreciated.

[bbatsov]

Can't help with that, but you can always rely on the CI instead if you can't set this up locally.

I'm not sure I like the name of the new face, but the spirit of the change is fine. Probably it should mentioned somewhere in the README as well, otherwise no one will likely find it.

[Ramblurr]

Can't help with that, but you can always rely on the CI instead if you can't set this up locally.

Ah ok, I'll make some edits then and push :)

I'm not sure I like the name of the new face,

I'm happy to name it whatever you want, any suggestions?

Probably it should mentioned somewhere in the README as well, otherwise no one will likely find it.

I looked around but there didn't seem to be an obvious place. The existing faces (clojure-keyword-face and clojure-character-face) aren't mentioned in the README either. Should I add a new section for the faces?

[bbatsov]

Should I add a new section for the faces?

Yeah, it can be somewhere under "Configuration".

[yuhan0]

I'd vote for keeping the name clojure-discard-face or perhaps clojure-discarded-face because that's the official name for the #_ reader macro (https://clojure.org/guides/weird_characters#_discard), even though various tooling all seem to have their own names for it (ignore, uneval, etc)

There might be a minor semantic mismatch since - depending on the customized value of clojure-comment-regexp - it will be be applied to (comment) and other forms as well, but I can address that in a followup PR

[Ramblurr]

Ok I've updated the README with a new section on the faces, and updated the font-locking tests to reflect the new face.

The emacs-master build target is failing, but the errors seem unrelated to my changes.

Let me know if you prefer if I squash the commits manually.