[HTML5] Add JavaScriptToolsEditorPlugin.

A new editor plugin, specific to HTML5, that provide some extra features
needed to make the editor usable on that platform.

For now, it adds a "Download project sources" option in the "Tool" menu,
so the user can download the work done as a zip file (from the browser
storage).

platform/javascript/SCsub

@@ -9,6 +9,7 @@ javascript_files = [
     "javascript_eval.cpp",
     "javascript_main.cpp",
     "os_javascript.cpp",
+    "api/javascript_tools_editor_plugin.cpp",
 ]
 
 build_targets = ["#bin/godot${PROGSUFFIX}.js", "#bin/godot${PROGSUFFIX}.wasm"]