refs https://github.com/nim-lang/Nim/pull/23873#discussion_r1687995060,
fixes#23386, fixes#23385, supersedes #23572
Turns the `nfOpenSym` node flag implemented in #23091 and extended in
#23102 and #23873, into a node kind `nkOpenSym` that forms a unary node
containing either `nkSym` or `nkOpenSymChoice`. Since this affects
macros working on generic proc AST, the node kind is now only generated
when the experimental switch `genericsOpenSym` is enabled, and a new
node flag `nfDisabledOpenSym` is set to the `nkSym` or `nkOpenSymChoice`
when the switch is not enabled so that we can give a warning.
Now that the experimental switch has more reasonable semantics, we
define `nimHasGenericsOpenSym2`.
