Introduce catch and new %error handler mode for resumable parsing

[sgraf812]

This is the "production-ready" version of #272 and #283, superseding both.

---

This patch enables users of happy to emit multiple parse errors in the same file and produce a partial syntax tree.

• The main feature is described in the user's guide entry.
• I also added a user's guide entry for the hitherto undocumented "expected token list" feature (which I broke to vastly improve it, but it was not documented anywhere). This will fix happyExpToks table can be larger than happyTable #271 because we no longer need an uncompressable table.
• The most important place documenting the implementation and the backwards compatibility dance with the existing error token is described in Note [Error recovery] and its children.

This patch unblocks our GSoC student @Kariiem's work over at https://gitlab.haskell.org/ghc/ghc/-/merge_requests/13145.

[Ericson2314]

This is very cool! :)

[Ericson2314]

Suggested change

	> = joinSymSets (\h ->fromMaybe (Set.singleton h) (lookup h env))
	> = joinSymSets (\h -> fromMaybe (Set.singleton h) (lookup h env))

[sgraf812]

Will merge soon. It would be good if anyone could give the user's guide entry a read and tell me if it made any sense.

[Ericson2314]

The users guide made sense, that was the only part I read.