Make `identifier` parse unparenthesized `->` (#18060)

This MR adds a case for unparenthesized -> to the identifier parser (exported as parseIdentifier) to fix #18060 (closed).

parseIdentifier is also (transitively) used in other GHCi commands, the debugger, and haddock. But it makes sense that anything that accepts both + and (->) should also accept ->.

The test checks that :i ~ works as well because it used to have the same issue.