Add from_xml_string to MuJoCo WASM bindings.

This change introduces new functions to load an `mjModel` from an XML string. Two overloads are provided: one that creates a temporary VFS and one that uses a MjVFS arg.

Also updated other places where `from_xml_string` should be used: demos, readme, etc.

PiperOrigin-RevId: 882581762
Change-Id: I8428196f8f56ae9630fcd4f0bb6763cdc1c43da0

wasm/demo_app/app.ts

@@ -205,10 +205,7 @@ class App {
   }
 
   loadModel(xmlContent: string) {
-    // Write xml as a file so that mujoco can find it
-    (mujoco as any).FS.writeFile('/working/model.xml', xmlContent);
-
-    this.mjModel = mujoco.MjModel.mj_loadXML('/working/model.xml');
+    this.mjModel = mujoco.MjModel.from_xml_string(xmlContent);
     if (!app.mjModel) {
       throw new Error('Failed to load model');
     }
@@ -437,8 +434,6 @@ function setupWindowEvents() {
   // Tip: put "window.dispatchEvent(new Event('unload'))" in the console to test
   window.addEventListener('unload', () => {
     app.dispose();
-
-    (mujoco as any).FS.unmount('/working');
   });
 
   window.addEventListener('keydown', (event) => {
@@ -464,10 +459,6 @@ async function main() {
   try {
     mujoco = await loadMujoco();
 
-    // Set up emscripten virtual file system
-    (mujoco as any).FS.mkdir('/working');
-    (mujoco as any).FS.mount((mujoco as any).MEMFS, {root: '.'}, '/working');
-
     app = new App();
 
     setupWindowEvents();