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/tests/sandbox/main.ts

@@ -20,9 +20,6 @@ declare function loadMujoco(): Promise<MainModule>;
 async function main() {
   const mujoco: MainModule = await loadMujoco();
 
-  (mujoco as any).FS.mkdir('/working');
-  (mujoco as any).FS.mount((mujoco as any).MEMFS, {root: '.'}, '/working');
-
   const xmlContent = `
     <mujoco model="Box falling">
       <option viscosity="1"/>
@@ -37,13 +34,12 @@ async function main() {
       </worldbody>
     </mujoco>`;
 
-  (mujoco as any).FS.writeFile('/working/hello.xml', xmlContent);
   let model: MjModel|undefined;
   let data: MjData|undefined;
 
   try {
     console.log('Hello world!: Loading model');
-    model = mujoco.MjModel.mj_loadXML('/working/hello.xml');
+    model = mujoco.MjModel.from_xml_string(xmlContent);
     if (!model) {
       throw new Error('Failed to load model');
     }
@@ -57,7 +53,6 @@ async function main() {
   } finally {
     model?.delete();
     data?.delete();
-    (mujoco as any).FS.unmount('/working');
   }
 }