templates: add .email_domain() method to Signatures

[ilyagr]

This is slightly less pretty than making an Email type, but I think is much easier.

I would use instead of the last argument of:

  concat(
    label("email", signature.username()),
    "@",
    label("email",
      signature.email().remove_prefix(
        signature.username()
      ).remove_prefix("@")
    )
  )

(Aside for more motivation: I initially had a bug above by using .remove_prefix(signature.username() ++ "@"). This is wrong since an email might not contain an @, as I discovered while looking at the code. The bug is now fixed, but signature.email_domain() would avoid the problem)

The effect I was going for (though I'm still not sure whether and how many characters I want to trim to):

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• n/a I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[yuja]

Maybe it's time to add an Email template type (and deprecate .username())? Suppose the goal is to label @ differently, an Email object could be rendered to <local>foo</local>@<domain>example.org</domain> by default.

[ilyagr]

That would be better, but also a lot more work. As I mentioned, this is giving me pause at the moment TBH. I haven't decided whether/when I want to do it.