chore: upgrade Emscripten (#729)

* chore: upgrade Emscripten

I do not have a particular reason for this.
Just hoping that this might bring some performance improvement,
perhaps.
I have tested it, but I have not noticed a significant difference.

* 4.0.18 appears to be the latest now

---------

Co-authored-by: Jack Slater <[email protected]>

Author: WofWca

.github/workflows/build.yml

@@ -60,8 +60,8 @@ jobs:
     - name: Install Dependencies
       run: |
         cd emsdk
-        ./emsdk install 3.1.58
-        ./emsdk activate 3.1.58
+        ./emsdk install 4.0.18
+        ./emsdk activate 4.0.18
     - name: Compile
       run: |
         echo I_ACKNOWLEDGE_THE_MAKEFILE_IS_DEPRECATED=1 > Makefile.local