feat: support format option for transform

[sastan]

This allows to use transform to output a different format without
going through build which would resolve all imports.

Import handling:

• cjs: imports are not resolved and re-written as require calls
• iife: any import will throw

[nathanforce]

Love to see this! Is there any way I can test this out against my project locally?

[sastan]

@nathanforce I have no published package. You need to check out the branch and use make platform-all to have a bundle.

[sastan]

@evanw Can i help with anything to get this PR merged? Very much open to feedback on this PR if you have suggestions for improvements.

[evanw]

Thank you for putting this PR together. However, this is sufficiently complicated and enough of a core part of esbuild that I don't think implementing this as an external contribution makes sense. Some examples of things that aren't handled here:

• Making this work the same way with transforming and with building without bundling
• Making sure transforming doesn't touch the file system
• Handling lazy exports from various non-JavaScript loaders

I have also already started work on implementing #109 and am most of the way there. I will make sure this works as a part of the work for #109.

[evanw]

Closing this as #109 has just been implemented.