Initial usage of async/await in JS library code. NFC (emscripten-core#23104)

Convert asyncLoad from promises to async/await.

This lays the groundwork for more a larger usage in emscripten-core#23068

Author: sbc100

src/library.js

@@ -2194,16 +2194,12 @@ addToLibrary({
     return x.startsWith('dynCall_') ? x : '_' + x;
   },
 
-  $asyncLoad: (url) => {
-    return new Promise((resolve, reject) => {
-      readAsync(url).then(
-        (arrayBuffer) => {
+  $asyncLoad: async (url) => {
+    var arrayBuffer = await readAsync(url);
   #if ASSERTIONS
-          assert(arrayBuffer, `Loading data file "${url}" failed (no arrayBuffer).`);
+    assert(arrayBuffer, `Loading data file "${url}" failed (no arrayBuffer).`);
   #endif
-          resolve(new Uint8Array(arrayBuffer));
-        }, reject);
-    });
+    return new Uint8Array(arrayBuffer);
   },
 
   $alignMemory: (size, alignment) => {

test/other/codesize/test_codesize_hello_dylink.gzsize

@@ -1 +1 @@
-6276
+6274

test/other/codesize/test_codesize_hello_dylink.jssize

@@ -1 +1 @@
-13831
+13816