Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

Throw Value/TypeError for invalid $bodies in imap_mail_compose()

Small drive by refactoring to use HashTables

Closes GH-6371

Fix #80223: imap_mail_compose() leaks envelope on malformed bodies

We have to clean up even on failure.

Closes GH-6322.