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/README.md

@@ -307,7 +307,7 @@ The function `mjv_updateScene` populates an `mjvScene` object with information
 from `mjModel` and `mjData`.
 ```typescript
 // Create instances of the necessary structs.
-const model = mujoco.MjModel.loadFromXML(xmlContent);
+const model = mujoco.MjModel.from_xml_string(xmlContent);
 const data = new mujoco.MjData(model);
 const scene = new mujoco.MjvScene(model, 1000);
 const option = new mujoco.MjvOption();