Runtime: Add nixInherit matcher in nix.vim syntax

Runtime: Add nixInherit matcher in nix.vim syntax

Perform the lookahead in `nixInheritAttributeScope`, then hand over to a
new region called `nixInheritAttributeSubExpr`, which sets the match
start to one char after the opening bracket to avoid a double-match.

Finally, only do a lookahead to `)` in `nixInheritAttributeSubExpr` (and
thus make sure the region is closed to not get a match of `nixParen`
here) and let `nixInheritAttributeScope` close the bracket.

Signed-off-by: Christian Brabandt <cb@256bit.org>