Add JSX spread children

[calebmer]

The JSX specification was just updated (here: facebook/jsx#59) to add for JSX spread children. See the rationale here: facebook/jsx#57

What would be involved in adding support for this new feature of the JSX specification to the TypeScript parser?

[mhegazy]

the change should not be that big to the parser, there are changes needs to the checker, and the emitter as well.
PRs are welcomed

[sandersn]

#11591 adds this feature.