Add ERC support

[TRSx80]

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ERC is the IRC client which is built-in to Emacs.

This should be a decently complete theme, personally I had only changed about 4 faces to get it to a usable state for myself, however before submitting I went above and beyond and set several more.

I did not, however, set any of the fg:erc-color-face[0-15] nor bg:erc-color-face[0-15], as I have never even seen those used anywhere (maybe on a terminal? but I am GUI user). There were a few more oddball ones I did not bother with either, but I don't think they are important and I never saw them used anywhere, either.

I took a couple screen shots:

N.B.: I am using some custom ELisp to assign different colors to different nicks, that is not part of this commit.

[TRSx80]

I did also use Nord colors for the custom nick coloring, I could make another PR and put that in contrib(?) or maybe just paste the code up somewhere if anyone is interested. It was basically just something I copped off EmacsWiki and modified a bit.