fs: adds FSConfig to replace experimental writefs (#1061)

This adds a new top-level type FSConfig, which is configured via
`ModuleConfig.WithFSConfig(fcfg)`. This implements read-only and
read-write directory mounts, something not formally supported before. It
also implements `WithFS` which adapts a normal `fs.FS`. For convenience,
we retain the old `ModuleConfig.WithFS` signature so as to not affect
existing users much. A new configuration for our emerging raw
filesystem, `FSConfig.WithSysfs()` will happen later without breaking
this API.

Here's an example:
```
moduleConfig = wazero.NewModuleConfig().
	// Make the current directory read-only accessible to the guest.
	WithReadOnlyDirMount(".", "/")
	// Make "/tmp/wasm" accessible to the guest as "/tmp".
	WithDirMount("/tmp/wasm", "/tmp")
```

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/sysfs/dirfs_test.go

@@ -380,5 +380,5 @@ func TestDirFS_TestFS(t *testing.T) {
 	testFS := NewDirFS(tmpDir)
 
 	// Run TestFS via the adapter
-	require.NoError(t, fstest.TestFS(&testFSAdapter{testFS}))
+	require.NoError(t, fstest.TestFS(testFS.(fs.FS)))
 }