Add early error to disallow invalid adjacent JSX elements about jsx

[Huxpro]

@nicolo-ribaudo Oh wow this is interesting. Thanks for reporting this and providing a very clear analysis.

Since we've moved the spec to ecmarkup (#135), I think we are finally in a good position to formalize errors like this. Would you mind making a PR?

Regarding the potential breaking change on the Flow parser: @pieterv, is Flow team comfortable with this change?

[pieterv]

Looks like this has been fixed in Flow since this issue has been created as the astexplorer link is showing the elements after the first JSX element parsed as binary expressions. Seems like a reasonable change anyway, we can fix any issues that come up.

See: https://astexplorer.net/#/gist/66ddd5a43c135ee13b7872e65de033c4/47ffc8a8b45cbbc6e8152423b9c7d5c351934288

[Huxpro]

@nicolo-ribaudo

I propose to disallow that code since it is highly confusing, similarly to how -1 ** 2 is disallowed by the ECMAScript specification.

How is this specified? I couldn't find any SS: early errors clause dedicated for that in the full spec or the original https://tc39.es/proposal-exponentiation-operator/ addition. I was imaging that we can specify JSXElement < similarly in #141