Add support for JS module blocks

[alangpierce]

JS module blocks is a TC39 proposal, currently stage 2, that allows multiple ES modules to be defined in the same file.

More details here: https://github.com/tc39/proposal-js-module-blocks

Example code:

let moduleBlock = module {
  export let y = 1;
};
let moduleExports = await import(moduleBlock);
assert(moduleExports.y === 1);

There are a few possible levels of supporting this syntax:

1. Parse the code as a regular block so that Sucrase can preserve it and transform the module contents. This is coming soon as part of porting Parse JS Module Blocks proposal babel/babel#12469 . This should handle things like the JSX transform, transforming TS enums, and removing types.
2. Update all file-wide logic to treat the module block as its own "file". This particularly comes up when determining name usage to remove type-only imports. It would also be necessary for the imports transform, though it may make sense to disallow the module syntax when transpiling to CommonJS.
3. Transpile the module block to work in a runtime that doesn't support it. This would likely involve somehow emitting multiple files for a single input file, so it seems probably best to call it out of scope for Sucrase.

This task will track tasks 1 and 2, though if others feel strongly that Sucrase should implement 3, let me know.