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.go

@@ -33,6 +33,11 @@ func (d *dirFS) String() string {
 	return d.dir
 }
 
+// Open implements the same method as documented on fs.FS
+func (d *dirFS) Open(name string) (fs.File, error) {
+	return fsOpen(d, name)
+}
+
 // OpenFile implements FS.OpenFile
 func (d *dirFS) OpenFile(name string, flag int, perm fs.FileMode) (fs.File, error) {
 	f, err := os.OpenFile(d.join(name), flag, perm)