Add compilation mode warning face

[jjwest]

This updates the compilation mode regex so that warnings are colored properly,
making them easier to differentiate from errors.

Before:
https://i.imgur.com/mChGm3k.png

After:
https://i.imgur.com/hBXjGk0.png

[rust-highfive]

r? @pnkfelix

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[pnkfelix]

Hmm I've played with something like this in the past but I don't think I had gotten the updates to the numbers right when I tried it.

Let me give it a whirl to double-check that the overall UX (namely for jumping from error to error) remains the same -- a big problem we want to avoid is forcing a user to step through tons of warnings when the intention was to step directly from the last diagnostic to the next true error.

[pnkfelix]

(I'm sorry that I haven't yet actually followed through on my promise to give this a whirl. Its still on my todo list.)

[pnkfelix]

I gave this a quick whirl locally. (No, I didn't. I thought I was reloading my rust-mode.el but I forgot I need to actually restart emacs; reloading the lisp code does not suffice.)

Outdated incorrect commentary follows in details block.

I would have thought that if I get both warnings and errors during a compilation (e.g. because I'm compiling multiple crates), I would see distinct increments to compilation-num-errors-found versus compilation-num-warnings-found.

~~But I am not seeing that here.

(Maybe that is for some reason not what you desired?)~~

I did see that we are at least flagging the warnings in the diagnostic output, and M-x compilation-next-error (often mapped to TAB) is jumping between them. But I'm still not sure what is best.

I will admit that in some (many?) projects, one often does not see warnings mixed with errors, because the compiler will only emit warnings for a given input crate if no errors were issued (right?)

So its possible the approach used here is still a net win and should be merged.

But I don't want to do so without first double-checking if there is someway to get emacs to account for the warning/error distinction (as I think should be reflected in the two aforementioned variables).

nikomatsakis do you have any thoughts here? Would you prefer to just merge this as is?

[pnkfelix]

(Or is the situation I just described where we already were at, and this PR is truly just an improvement on the status quo... hmm sorry if I misunderstood where we are in rust-mode.el at the moment...)

[pnkfelix]

Yes okay looking at the linked images I am realizing that something must be wrong with my own (screen terminal-based) setup, since I am not seeing quite the same set of colors that are shown there.

[pnkfelix]

So I'm going to go ahead and merge since this does seem to be a net win.