[Issue 23797] Improve type-testing `is` expressions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23797

--- Comment #1 from Bolpat <qs.il.paperinik at gmail.com> ---
As for the right-hand side of `is(Type == Keyword)` for function pointer and
delegate types, it also makes sense to ask if they’re carrying attributes. The
keyword attributes should be allowed, but I think also user-defined ones should
be allowed. Also visibility can be allowed for any symbols.

The same for the simplified `is …` syntax. `DG is @nogc` is clear to read.

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