added support for esbuild loader option

[Flammae]

Adds support for a new esbuild.loader option.

Currently, loader cannot be configured and is guessed by the file extension. One possible use case is allowing users to use jsx loader for .js files.

[Flammae]

I'm not sure which one's more desired:
a. Passing the result of fs.readFile to the loader functions code argument or
b. Passing the result of plugin.transform which we already have by the time loader is returned.
In this version I decided to go with the second option.

[Flammae]

Hey @sxzz, any updates/instructions on this?

If you are undecided on my previous comment, we could just merge the first version of loader?: Loader (constant value, no function) and wait for the feedback. That way a chunk of the use case can be addressed instantly. And if a function is ever implemented it won't be a breaking change