wasi: replaces existing filesystem apis with fs.FS

[codefromthecrypt]

This changes SysConfig.WithFS and SysConfig.WithWorkDirFS to accept an fs.FS for the root ("/") and working directory (".") file systems. This allows end users to leverage standard libraries that adhere to this, including OS, HTTP and embedded filesystems.

For example, here's how you can allow WebAssembly modules to read
"/work/home/a.txt" as "/a.txt" or "./a.txt":

wasi, err := r.InstantiateModule(wazero.WASISnapshotPreview1())
defer wasi.Close()

sysConfig := wazero.NewSysConfig().WithFS(os.DirFS("/work/home"))
module, err := wazero.StartWASICommandWithConfig(r, compiled, sysConfig)
defer module.Close()
...

Note: Further development will happen after this, for example to allow creation of new directories, though priority of that is demand-based. If you need further FS features, you should ask on #390 with your use-case!

[codefromthecrypt]

ok this is rebased. next steps before ready for review:

1. fix whatever's going on in the write tests in windows
2. Test_Cat is the only thing actually using this. Add tests to ensure working directory and root directory paths work
3. scrub for TODOs and fix here, leave them or open a new issue.

[codefromthecrypt]

opened #408 to follow-up at some point and add another kind of integration test, now that it is easy to compare against an fs.FS

[codefromthecrypt]

took some reading to discover there are no plans for an actual chrooted FS. Until then, folks with high isolation needs will have to write their own fs.FS impl (or find some library that isn't abandoned that tries to do this!)

[codefromthecrypt]

cc @pims this is an API break, but a good one as we've gotten rid of an incompatible FS and formalized the API. Hope this doesn't drift you too much!

[pims]

Thanks for the heads-up @codefromthecrypt

This looks like a very sensible change and until 1.0 is released, no stress about API breaking changes.