Add comments to javascript wrapper parts.

The code in pre.js and engine.js is a bit confusing to see in isolation,
since the files aren't valid JS files by themselves. This just adds some
explanatory text to both files.

Fixes #22937.

(cherry picked from commit 61d5513525)

platform/javascript/pre.js

@@ -1,2 +1,5 @@
 var Engine = {
 	RuntimeEnvironment: function(Module, exposedLibs) {
+		// The above is concatenated with generated code, and acts as the start of
+		// a wrapper for said code. See engine.js for the other part of the
+		// wrapper.