PHP: Do not pull WebGL in Playground web (#2318)

## Motivation for the change, related issues

Only uses `-s MAIN_MODULE` when building for Node.js+JSPI to fix a crash
in the web version of Playground.

#2248 added `-s MAIN_MODULE` to the main `emcc` call linking the final
PHP.wasm file. It worked fine in Node.js, but on the web, after
rebuilding with `recompile:php:web:jspi:8.4`, Playground threw a
`ReferenceError: document is not defined`.

It turn out, `-s MAIN_MODULE` pulls in a pile of extra libraries in case
some dynamically loaded lib needs them. One of them was
[webgl](https://github.com/emscripten-core/emscripten/blob/b0c461c10482e12f88e192e04b3640acb1577f64/src/lib/libhtml5_webgl.js#L423)
that assumes `globalThis.document` exists – hence the error. See [this
Emscripten
thread](https://groups.google.com/g/emscripten-discuss/c/nz2rXAp_vuQ?pli=1)
for more details.

## Implementation

Moves `-s MAIN_MODULE` to platform-only flags for Node.js+JSPI.

I've tried switching to `-s MAIN_MODULE=2` as suggested in the mailing
lists. It fixes it for the web build, but it breaks loading XDebug.so in
Node. The error says `missing export type for core_globals`. I assume we
need to pull in some additional dependencies and declare them
explicitly.

## Remaining work

Rebuild all the PHP versions – Node.js and Web – and confirm the tests
pass

---

Unrelated – I've noticed to patch `with-xdebug.ts` as follows to be able
to run it from source:

```
diff --git a/packages/php-wasm/node/src/lib/xdebug/with-xdebug.ts b/packages/php-wasm/node/src/lib/xdebug/with-xdebug.ts
index 3b40aa2..41fec3d237 100644
--- a/packages/php-wasm/node/src/lib/xdebug/with-xdebug.ts
+++ b/packages/php-wasm/node/src/lib/xdebug/with-xdebug.ts
@@ -17,7 +17,11 @@ export async function withXdebug(
        }
 
        const fileName = 'xdebug.so';
-       const filePath = await getXdebugExtensionModule(version);
+       let filePath = await getXdebugExtensionModule(version);
+       if (filePath.startsWith('file://')) {
+               filePath = filePath.substring(7);
+       }
+
        const extension = fs.readFileSync(filePath);
 
        return {
```

cc @mho22 @zaerl

---------

Co-authored-by: Francesco Bigiarini <[email protected]>

Author: adamziel