[WIP] feat(build): allow specifying multiple targets

[hobofan]

Addresses #157 / #469

There are still a few things that should be done:

• Deduplicate typescript definitions
• Refactor https://github.com/hobofan/wasm-pack/blob/d641e017e3c5869c8749b03b7b9c617bc749cb1b/src/manifest/mod.rs#L373-L390 into own function
• Tests tests tests ❗️

A few things that would be great to get your feedback on @ashleygwilliams :

• I removed the EsModulesPackage, etc. and flattened that to the NpmPackage, as I there was quite some duplication there, and I didn't see a good way to achieve the feature without that. I hope that's okay 😬 .
• Should I also tackle give option target=all which generates pkg supporting commonjs and esm #313 and refactor target to use an enum #332 in this PR, or better in a different PR?
• I'm not sure if I should add a special case handling for the case of a single target, in which case the old file naming convention ({crate_name}.js) should be used instead of the new names ({crate_name}_{target}.js)? I guess that might be required for better compatibility?

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[ashleygwilliams]

hey @hobofan ! thanks for this- it's late here for me but i'll take a look and leave some comments tomorrow morning. for a feature like this, one thing that's gonna be important is the expected final structure of the "isomorphic" package. could you share here in a comment the structure you anticipate?

thanks again!

[ashleygwilliams]

@hobofan did you have a chance to read my previous comment? looks like we have some merge conflicts now... let me know where you are at with this PR and if there is anything i can do to help!

[ashleygwilliams]

closing as out of date. @hobofan if you'd like to tackle this still, feel free to reopen this PR or make a new one!