Add support for custom JSX pragmas

[alangpierce]

Fixes #207

As with Babel, we assume the pragma is a dot-separated list of identifiers (or
just an identifier), which we parse so that we can replace the base name if
necessary.

[StreetStrider]

Hello. Is pragma comment supported?

[alangpierce]

@StreetStrider No, not at the moment. I just filed #479 to track the issue. What's your use case, out of curiosity? Do you have a codebase where different files need different JSX behavior?

[StreetStrider]

@alangpierce I haven't got specific use-case right now, I'm learning things here. I just previously knew that jsx pragma is a valid way to setup a vdom constructor. I'm not sure about real user base, but if there is such a way, this pragma will occur in real world JSX.

I think my future use-case would be so: explicitly use jsx pragma comment in every file and explicitly import corresponding vdom constructor. It is explicit and does not depend on external config of any kind. However, this is a still thing to investigate for me.

[alangpierce]

@StreetStrider Makes sense. Yeah, JSX is a bit awkward in that it started out as a React thing and was made more general, so you need custom configuration (or pragma comments) to configure the transpile output. Seems like a roadblock making it even less likely that it'll eventually get native browser support.

I think in practice it tends to happen for projects using two different UI frameworks, e.g. a project that's in a transition between React and Vue. But it's true that pragma comments have the nice aspect of making code more universal and less reliant on external config.

[StreetStrider]

@alangpierce I will be notified here. However, if this feature is too complex for Sucrase, I'm OK if it will not be implemented.
I'm attracted by simplicity of Sucrase's usage. Right now I'm using tsc + noEmit to typecheck + Sucrase to cut TypeScript syntax (previously, same for Flow). So, I'm always produce JS with Rollup + plugin infrastructure; I find that is a robust approach (I'm not using tsc for bundling, or Babel, or anything else — it's always Rollup, so I can freely reuse configs, plugins and knowledge).

Hopefully, JSX can be resolved in Sucrase, but if not, I will investigate rollup-plugin-jsx as a second step. I'm pretty sure it supports all the features, but involves Babel internally as a drawback.