never patterns: Document behavior of never patterns with macros-by-example
`never_patterns` makes `!` parse as a pattern so I was worried about breaking macros-by-example matching. Turns out we're fine because the cases that now match `$p:pat` used to error in the past. The only tricky case is `!` by itself, which backwards-compatibly doesn't match `$p:pat`. I have no idea why tho, I didn't think of that when I was implementing parsing 😅.
This adds tests so we don't regress the current behavior.
r? `@compiler-errors`
a060ed2c06