Better handling of invalid identifiers

[jehna]

Originally from another thread:

instead of just forcing an invalid suggestion to be valid (with toIdentifier), we could detect that it's invalid (with isValidIdentifier) and then provide that feedback to the LLM, asking it to give a new suggestion; probably with some max retry limit; after which we could fall back to using the invalid suggestion run through toIdentifier, or log a warning and leave it un-renamed or similar.

Originally posted by @0xdevalias in #117 (comment)