Disallow < after a JSX element

[nicolo-ribaudo]

Fixes #120.

This was quite hard to specify:

• I initially tried to add an early error to RelationalExpression saying something like "It is a SyntaxError if the left side of the expression recursively contains a JSXElementOrFragment as its right-most descendent.", but it would have required writing some custom sdo for every production that can derive a JSXElementOrFragment as a child of RelationalExpression.
• The spec explicitly ignores lookaheads in static semantics, so I wanted to avoid doing it.

⚠️ Almost all the tools already implement this error, but it would be a breaking change for the Flow type checker.

[Huxpro]

I'm still waiting for Flow team to give a response but other than that (and a question), this LGTM.

[Huxpro]

Do we need this production to be able to define an early error on PrimaryExpression :: JSXElementOrFragment <?

[nicolo-ribaudo]

We could do something like

PrimaryExpression :: JSXElementOrFragment

- It is a Syntax Error is this production is followed by the `<` token

but I used that new production to avoid introducing new language/checks that are not already used in the ecma262 spec

[Huxpro]

I understand that without PrimaryExpression :: JSXElementOrFragment < we don't have a production to match with. But it's also not ideal to me to only add this production specifically for the early error 🤔

[nicolo-ribaudo]

Note that the ecma262 spec already does this in a few cases, for example to disallow a?.b`c`: https://tc39.es/ecma262/#sec-left-hand-side-expressions-static-semantics-early-errors

[Huxpro]

I'm still on board with fixing #140 but I think we need to find a better mechanics.

[Huxpro]

Came across and I think this LGTM.

It may be nicer if we could have a note like https://tc39.es/ecma262/#sec-left-hand-side-expressions-static-semantics-early-errors to say something similar:

This production (only) exists in order to prevent ...

As I'm no longer employed by Meta, we will need some help to actually merge this. cc @poteto if anyone on the team is interested in taking a look.