Implement \cases (Proposal 302)

This merge request implements Proposal 302: \cases - Multi-way lambda expressions.

See also haddock!4 (merged), which adds the haddock changes required for this MR.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• update user guide
• fix core lint error for empty case

Closes #20768 (closed)