Rendering to multiple backends

[samhh]

At the moment you can specify an output "backend", but you might want multiple.

If/when we support multiple filetypes (we don't/wouldn't really yet as .ts is subsumed by .tsx) we can output multiple sibling files, but what about distinct backends for the same filetype?

Say, given "backends": ["ts", "tsx"], what should the output be? Because at present the export names would conflict.

[samhh]

Note also that js, jsx, and dts backends would now be fairly trivial to add, particularly if we changed the TypeScript output to be simply js(x) and dts.