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>

fsconfig_example_test.go

@@ -0,0 +1,27 @@
+package wazero_test
+
+import (
+	"embed"
+	"io/fs"
+	"log"
+
+	"github.com/tetratelabs/wazero"
+)
+
+//go:embed testdata/index.html
+var testdataIndex embed.FS
+
+var moduleConfig wazero.ModuleConfig
+
+// This example shows how to configure an embed.FS.
+func Example_withFSConfig_embedFS() {
+	// Strip the embedded path testdata/
+	rooted, err := fs.Sub(testdataIndex, "testdata")
+	if err != nil {
+		log.Panicln(err)
+	}
+
+	moduleConfig = wazero.NewModuleConfig().
+		// Make "index.html" accessible to the guest as "/index.html".
+		WithFSConfig(wazero.NewFSConfig().WithFSMount(rooted, "/"))
+}