Introduce a `:*@foo` slurpy named argument syntax

[lizmat]

Inspired by rakudo/rakudo#2797

This would allow:

% raku -e 'sub foo(:@bar) { dd @bar }; foo :bar<a>' 
Type check failed in binding to parameter '@bar'; expected Positional but got Str ("a")
  in sub foo at -e line 1

to just work like:

% raku -e 'sub foo(:@bar) { dd @bar }; foo :bar<a b>'
("a", "b")

The syntax is currently a compilation error:

% raku -e 'sub foo(:*@bar) { dd @bar }; foo :bar<a>' 
===SORRY!=== Error while compiling -e
Missing block
at -e:1
------> sub foo(:⏏*@bar) { dd @bar }; foo :bar<a>

Disadvantages: apart from the grammar work, this would also need quite a bit of work in the binder. And the user would need to know to specify :*@foo instead of :@foo.

Advantages: would be generally useful, and would allow the "adapt the Capture hack" in MAIN handling to be removed.

[Leont]

How would this interact with typed variables? Would that just not be allowed?

[lizmat]

Adding type checking to slurpy arrays in general should be doable within the new Raku grammar and RakuAST.

But if that wouldn't happen, then yes the same limitation would apply, I'd say.

[landyacht]

Would there also be **- and +-type slurpies of this sort, e.g. for controlling behavior for parameters like :foo(@bar, 'baz') or :foo(@bar, @baz)?

Ref. https://docs.raku.org/language/signatures#Types_of_slurpy_array_parameters

[codesections]

I'm don't have a well-thought-out view on whether this change is worthwhile, but I'll mention that

sub foo(:@bar) { dd @bar }; 
foo :bar["a"];

already works, which slightly reduces the use-case for this new syntax outside of MAIN/cli argument parsing. (Might still be worth adding, though)