Demo: Only include DASH_WASM parser when explicitly asked for

In our demo, we have specific scripts to start that demo with our
WebAssembly MPD parser, the `DASH_WASM` feature.

What those do is to set a `__INCLUDE_WASM_PARSER__` boolean to
`true` in global scope, which signals to the demo page that it should
include the `DASH_WASM` feature and load the corresponding WebAssembly
file.

However I noticed that when enabling "multithreading" mode in our demo
page, it tried to download the WebAssembly MPD parser regardless of the
`__INCLUDE_WASM_PARSER__` boolean.

This made sense pre-`v4.1.0` when we only relied on the WebAssembly
parser to parse a DASH MPD in "multithreading" mode. But now that we
also have a JS parser embedded inside our worker code, it seems
unnecessary.

It also led to some http 404 when testing the demo locally without
having built the WebAssembly file. Those don't matter much though
because the RxPlayer automatically fallbacks to the JS parser.

Also, the demo built on our GitHub pages accessible on
https://developers.canal-plus.com/rx-player/ do have the WebAssembly
MPD parser file built. This means that when enabling the
"multithreading" mode on that demo, we're actually loading and using
that WebAssembly file to parse MPDs.

Doing the change I'm proposing here of only using the WebAssembly parser
if `__INCLUDE_WASM_PARSER__` is set to `true` will thus lead to a small
change in the globally-accessible demo page, where we will be using the
default JS parser instead in "multithreading" mode.

We could also in the future add a checkbox in our demo specifically for
the inclusion or not of the WebAssembly parser.

demo/scripts/modules/player/index.ts

@@ -455,7 +455,7 @@ const PlayerModule = declareModule(
       player
         .attachWorker({
           workerUrl: "./worker.js",
-          dashWasmUrl: "./mpd-parser.wasm",
+          dashWasmUrl: __INCLUDE_WASM_PARSER__ ? "./mpd-parser.wasm" : undefined,
         })
         .catch((err) => {
           // eslint-disable-next-line no-console