[HTML5] Port JavaScript inline code to libraries.

The API is implemented in javascript, and generates C functions that can
be called from godot.
This allows much cleaner code replacing all `EM_ASM` calls in our C++
code with plain C function calls.
This also gets rid of few hacks and comes with few optimizations (e.g.
custom cursor shapes should be much faster now).

modules/websocket/SCsub

@@ -3,11 +3,13 @@
 Import("env")
 Import("env_modules")
 
-# Thirdparty source files
-
 env_ws = env_modules.Clone()
 
-if env["builtin_wslay"] and not env["platform"] == "javascript":  # already builtin for javascript
+if env["platform"] == "javascript":
+    # Our JavaScript/C++ interface.
+    env.AddJSLibraries(["library_godot_websocket.js"])
+elif env["builtin_wslay"]:
+    # Thirdparty source files
     wslay_dir = "#thirdparty/wslay/"
     wslay_sources = [
         "wslay_net.c",