Emulate new JSX transform benefits

[fs-c]

I really like the new JSX transform because having a (deceivingly) "unused" import sit at the top of every file annoyed me to no end. I don't really care about the other changes of the new transform (see the post linked above) so the import is what I will focus on.

esbuild doesn't seem likely to introduce proper support for the new transform (evanw/esbuild#334) but they suggest either a plugin (which calls babel, so horrible performance) or to just use the inject option to inject the import into each file.

I'll submit a PR with the latter option in a second.