Parser: accept exprs like `$$0++--`.

Title. Isomorphic expressions also fail to parse. These should parse the same way we currently accept $($0++)--. This is a bug because GNU AWK does accept them, but very amusingly, their docs give it as an example of an incorrect expression:

Also, operators cannot be combined in a way that violates the precedence rules; for example, $$0++-- is not a valid expression because the first $ has higher precedence than the ++; to avoid the problem the expression can be rewritten as $($0++)--.

Thus, and if possible, I would like the patch to be easily revertible.

Start by locating the Rust parser entry points that handle prefix $ and postfix ++/--, then compare their behavior with the already accepted $($0++)-- form. Add regression coverage for $$0++-- and verify that it parses successfully while preserving existing precedence behavior; keep the parser change easy to revert.