Support binding to non-local vars in pattern matching

[palkan]

Examples:

case %w[a b c]
in [@a, $b, *@@rest]
end

@a #=> 'a'
$b #=> 'b'
@@rest #=> ['c']

Using vars in alt pattern is prohibited (similarly to the current behaviour for local vars).

Implements https://bugs.ruby-lang.org/issues/18408.

P.S. This brings back rightward assignment: 42 => @v.

[eregon]

Could you add specs for this in spec/ruby/language/pattern_matching_spec.rb ?

[palkan]

@eregon Done!