[v2] Infer assetDir from embed.FS

AssetDir is now inferred from the assets, if the assets is an
embed.FS, by taking the relativ path to the index.html joined with
the project root.

The assetDir flag still exists and can be used if the inferring doesn't
work, because the provided embed.FS wasn't defined in the main
package.

v2/internal/frontend/devserver/devserver.go

@@ -49,6 +49,11 @@ func (d *DevWebServer) WindowReload() {
 func (d *DevWebServer) Run(ctx context.Context) error {
 	d.ctx = ctx
 
+	assetdir, _ := ctx.Value("assetdir").(string)
+	d.server.Get("/wails/assetdir", func(fctx *fiber.Ctx) error {
+		return fctx.SendString(assetdir)
+	})
+
 	d.server.Get("/wails/reload", func(fctx *fiber.Ctx) error {
 		d.WindowReload()
 		d.desktopFrontend.WindowReload()