Ability to get esbuild output in scripts without reading from FS

[timocov]

esbuild is awesome and ultra fast tool to bundle JS code, but currently it doesn't do some minifications other tools like terser do (see #218).

esbuild has JS API so we can write simple script to run esbuild and then provide output to terser (for instance), but it seems that there is no option to run esbuild and get output from it without reading outfile manually, so you have to run esbuild first, then read out file and pass it to terser.

It would be nice to return the output from esbuild as returned value (even without writing the result on the disk totally) so we don't need to read it again from FS.

[evanw]

I agree that this would be useful. It would also bring the JavaScript API up to parity with the Go API, which can already do this. This is already an existing feature request though, so I'm going to close this as a duplicate of #139.