config: introduce default `at = @` revset alias

[thoughtpolice]

When using Windows, @ is a bit of an annoyance to type in the terminal due to Windows Terminal escape rules, so you actually have to write it with a backtick in front. It's annoying to remember this and not possible to configure a different behavior, as far as I can see.

However, there is a smart trick you can do to make this a lot easier: just alias @ to the name at and be happy. Because:

• It's short: sweet and easy to read, and requires no backticks.
• It's unambiguous: a is from commit alphabet, but t is from the change alphabet
• It can be typed without a huge loss in efficiency (and one hand, assuming QWERTY).

I believe it was Stephen Jennings who came up with this trick first in Discord, and I have it in my aliases, but I run into it every time I try to use JJ on a fresh Windows VM to test stuff. Let's make life a little easier for everyone and just ship this by default.

Checklist

If applicable:

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

[thoughtpolice]

There's no changelog or docs yet but I'll get to it shortly assuming nobody has strong feelings on this.

[jennings]

I am honored!

but z is from the change alphabet

"z"? 🤔