[SPLIT_MODULE] Support multi-split loading (#25571)

Currently, if the primary module name is `test.wasm` and when a
placeholder function is called, the emscripten JS runtime loads
`test.deferred.wasm`, which is hardcoded. But as we want emscripten to
support multi-split, the runtime has to know which secondary module to
load.

Binaryen's WebAssembly/binaryen#7975 changes the
placeholder import module naming scheme. Currently placeholder's import
modules are all `placeholder`:
https://github.com/WebAssembly/binaryen/blob/dcc704ce20e4723ae42012039bdb3b84ec2035f9/test/lit/wasm-split/multi-split.wast#L291-L295

But the Binaryen's companion PR will change this to the format of
`placeholder.[secondaryName]`. Then Emscripten runtime will parse this
module name to learn which secondary file to load. For example, if the
import module is `placeholder.2` and the import base is `3` and the
primary wasm file is `test.wasm`,
```wast
(import "placeholder.2" "3" (func $placeholder_3 (result i32)))
```
when `placeholder_3` function is loaded, the runtime will parse
`placeholder.2` and correctly figure out that it should load
`test.2.wasm`.

This PR still supports the old `placeholder` namespace, because this PR
has to land first for the Binaryen companion PR to land. After it lands,
we can remove the `placeholder` handling part.

Companion PR: WebAssembly/binaryen#7975

Author: aheejin

src/preamble.js

@@ -490,20 +490,25 @@ async function getWasmBinary(binaryFile) {
 var splitModuleProxyHandler = {
   get(target, moduleName, receiver) {
     if (moduleName.startsWith('placeholder')) {
+      let secondaryFile;
+      if (moduleName == 'placeholder') { // old format
+        secondaryFile = wasmBinaryFile.slice(0, -5) + '.deferred.wasm';
+      } else { // new format
+        let moduleID = moduleName.split('.')[1];
+        secondaryFile = wasmBinaryFile.slice(0, -5) + '.' + moduleID + '.wasm';
+      }
       return new Proxy({}, {
         get(target, base, receiver) {
           return (...args) => {
 #if ASYNCIFY == 2
             throw new Error('Placeholder function "' + base + '" should not be called when using JSPI.');
 #else
-            // TODO: Implement multi-split module loading
 #if RUNTIME_DEBUG
             dbg(`placeholder function called: ${base}`);
 #endif
             var imports = {'primary': wasmExports};
             // Replace '.wasm' suffix with '.deferred.wasm'.
-            var deferred = wasmBinaryFile.slice(0, -5) + '.deferred.wasm'
-            loadSplitModule(deferred, imports, base);
+            loadSplitModule(secondaryFile, imports, base);
 #if RUNTIME_DEBUG
             dbg('instantiated deferred module, continuing');
 #endif