* Add methods `Invulnerable() bool` and `SetInvulnerable(bool)` to the
Actor API accessible in JavaScript (e.g. `Self.SetInvulnerable(true)`)
* The Anvil is invulnerable - when played as, it can crush other mobs by
jumping on them but is not defeated by those mobs at the same time.
* Anvils don't destroy invulnerable mobs, such as other Anvils.
* Bugfix: the Electric Door is considered to be opened from the first
frame of animation when the door begins opening, and remains opened
until the final frame of animation when it is closing.
* New cheat code: `megaton weight` to play as the Anvil by default.
* If the player runs the PlayAsBird cheat they shouldn't be able to win
a high score on a level, so at level startup it detects whether the
DefaultPlayerCharacterDoodad has changed from default on a level that
doesn't use the Start Flag to set a specific doodad - and immediately
marks the session as cheated
