Improved parse errors

[SquidDev]

Menhir offers a fantastic system for reporting errors and we make pretty good use of it. However, there's some cases where we could provide better error messages. For instances:

• x = in an expression context should suggest ==.
• Specialised errors for do in an if block, then in a loop.
• Say which block is missing an end (or closing bracket).

We might also be able to implement error recovery, but I think that'd be a little harder.

I think the "best" way to do this is as follows:

• We remove all the messages in messages.txt, and replace them with identifiers (for instance UnknownAfterSimpleExpr).
• messages.txt gets compiled to messages.ml.strings.
• We parse this file, and generate a type definition t containing all these identifiers and a function to map state -> t.
• Then move messages into a separate file/function which accepts the current parser state and t and reports an error.

It'd hardly elegant, and involves some wonderfully silly meta-programming, but it should work.

[SquidDev]

Just linking some of the comments in this post. Some potentially useful things we could try.

[SquidDev]

The alternative strategy here is to use TreeSitter, though this comes with the obvious problem of having to link native code (and breaking our JS integration).

[SquidDev]

I think this was mostly addressed by cd06555892d353f8dd6672c484d6455dac4c8ee5. There's always room for improvement here, but I think that's less well scoped.