Allow using a custom loader

[tiennou]

Rollup Plugin Name: rollup-wasm

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes
• no

Breaking Changes?

• yes
• no

Description

This deprecates the targetEnv option and replaces it with loader which performs the same thing while also accepting an actual function.

When that happens, that function's body will be inserted in lieu of the default one, giving complete control over the way WASM is loaded into the runtime.

I need it because I'm trying to load code in a sandboxed Node 10 environment, which doesn't have process nor Buffer available, and that felt like the cleanest way to do that without giving you a bunch of hardcoded code that's meaningless outside of it.

[tiennou]

My initial attempt at this was to give a function name from the bundle instead of something that would only live in the config. But I couldn't find a way to get the mangling done by Rollup to apply to it from inside the plugin; that's why I settled on having a real function.

In other words, if I had set loader: 'myCustomLoader', that would translate to something like this in the final bundle:

function wasm (imports) { return myCustomLoader(0, null, 'AGFzbQEAAAABBwFgAn9/AX8DAgEABQMBAAAHEAIDYWRkAAAGbWVtb3J5AgAKCQEHACAAIAFqCwAkEHNvdXJjZU1hcHBpbmdVUkwSLi9yZWxlYXNlLndhc20ubWFw', imports) }

/* somewhere else */

function myCustomLoader$1(sync, path, src, imports) { /* … */ }

and that $1 prefix would have been a problem.