Support build / run under wasm / gopherjs

[paralin]

Uses a in-memory tree when osfs would otherwise be used.

[mcuadros]

Since in go-git, you can open and clone a repository declaring the kind of fs, I rather having this implementation as an implementation that returns "Not Implemented" errors for every method or one that simply panics if you call to os.New()

[paralin]

The bug / compile failure this is fixing is this:

paralin@forge go-git % GOOS=js GOARCH=wasm go build -v
package github.com/go-git/go-git/v5
        imports github.com/go-git/go-billy/v5/osfs
        imports golang.org/x/sys/unix: build constraints exclude all Go files in /home/paralin/go/pkg/mod/golang.org/x/sys@v0.0.0-20210415045647-66c3f260301c/unix

You can't import x/sys/unix without causing issues in wasm due to the build constraints.

In any case, it's necessary to exclude these from being imported on js, and there's already conditional platform files for all the other platforms, so why not js also?

If you have a example in go-git (which is what I ran above) and it doesn't work the same way in js without adding additional conditional flags, or doesn't compile at all, or unexpectedly throws errors, why is that better than just using a memfs to shim it?

[mcuadros]

Ideally, we should use https://github.com/WebAssembly/wasi-filesystem in the feature but since wasm_exec.js doesn't make use of this, this looks like the best solution.