Core: Fixed race condition caused by deferring the script

[RunDevelopment]

As discussed in #2102.

I'll make a different PR for the currentScript issue.

[RunDevelopment]

You can verify that this indeed fixes the race condition by replacing prism.min.js in the test page by @BrainStone with <script defer src="http://localhost/prism.js"></script> (localhost being this PR).

You'll see that the race condition is no more.

[mAAdhaTTah]

Def going to want a code comment in there explaining a bit of why this is necessary.

[RunDevelopment]

Comment as requested.
I also added readyState === 'interactive' to the condition because if the readyState === 'complete' then the script can be deferred all it wants, DOMContentLoaded has already been fired.