New observed flag to enable global MutationObserver

[hmans]

• Introduces a new observed attribute on <three-game>
• When enabled, the game will launch an extra MutationObserver to monitor attribute changes anywhere within its DOM.
• A warning is logged to remind the user that this is intended for development only, and should not be used in production projects because of the detrimental performance implications of using said MutationObserver.
• The test is a bit basic, but there isn't much we can realistically test, since this only kicks in when an actual human changes an attribute in their devtools.

Will fix #44 when merged.

[RangerMauve]

Woot! Thank you. 💜