syscall/js: enable a JS Promise pattern for Funcs

[TiemenSch]

Methods using js.PromiseOf return a js.Func that is callable using async/await in JS.

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[TiemenSch]

I was working with Go and WASM today, and registering functions with

js.Global().Set("someAlias",  FuncOf(someFunc))

works, but without error handling the program quickly exits after one wrong call from the JS side to global.someAlias().

This PR proposes to adopt a pattern that is already in use in the community to resolve requests to the Go code with promises, and could be useable by:

js.Global().Set("someAlias", PromiseOf(somePromiseAbleFunc))

where the only difference should be is that somePromiseAbleFunc should return an interface and error, which it can then use to either resolve or reject the promise safely with await global.someAlias().

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[earthboundkid]

I saw this in https://brunoluiz.net/blog/2022/aug/imagewand-privacy-first-image-conversion-experiment-with-golang-and-wasm/. I've written basically the same helper in my own WASM project, so I agree this serves a very common need, but it's new API, so you need to go through the Go proposal process. Go to the Issue tab and create a new issue called "proposal: syscall/js: add PromiseOf" and start a discussion there. If approved, your PR can be merged afterwards.