HTML5 start-up overhaul

- Implement promise-based JS interface for custom HTML page
   integration
 - Add download progress callback
 - Add progress bar and indeterminate spinner to default HTML page
 - Try downloading files multiple times when failing
 - Get rid of godotfs.js
 - Separate steps for engine initialization, game initialization and game
   start
 - Allow multiple games on one HTML page
 - Substitution placeholders only used in .html file
 - Placeholders renamed: $GODOT_BASE => $GODOT_BASENAME,
   $GODOT_TMEM -> $GODOT_TOTAL_MEMORY
 - Emscripten Module is now Engine.RuntimeEnvironment (no longer a global)

platform/javascript/detect.py

@@ -100,6 +100,7 @@ def configure(env):
 
     ## Link flags
 
+    env.Append(LINKFLAGS=['-s', 'EXTRA_EXPORTED_RUNTIME_METHODS="[\'FS\']"'])
     env.Append(LINKFLAGS=['-s', 'USE_WEBGL2=1'])
 
     if (env['wasm'] == 'yes'):
@@ -112,6 +113,7 @@ def configure(env):
     else:
         env.Append(LINKFLAGS=['-s', 'ASM_JS=1'])
         env.Append(LINKFLAGS=['--separate-asm'])
+        env.Append(LINKFLAGS=['--memory-init-file', '1'])
 
     # TODO: Move that to opus module's config
     if("module_opus_enabled" in env and env["module_opus_enabled"] != "no"):