[js/web] allow relative path matching (#21657)

### Description
<!-- Describe your changes. -->

This change allows to match external data path like `a.data` to
`./a.data`.


<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

onnxruntime/wasm/pre.js

@@ -15,6 +15,9 @@
  * @param {Uint8Array} externalDataFilesData
  */
 Module['mountExternalData'] = (externalDataFilePath, externalDataFileData) => {
+  if (externalDataFilePath.startsWith('./')) {
+    externalDataFilePath = externalDataFilePath.substring(2);
+  }
   const files = Module.MountedFiles || (Module.MountedFiles = new Map());
   files.set(externalDataFilePath, externalDataFileData);
 };