* 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.
* Switch from otto to goja for JavaScript engine.
* goja supports many ES6 syntax features like arrow functions,
const/let, for-of with more coming soon.
* Same great features as otto, more modern environment for doodads!
* The Anvil doodad is affected by gravity and becomes dangerous when
falling. If it lands on the player character, you die! If it lands on
any other mobile doodad, it destroys it! It can land on solid doodads
such as the Electric Trapdoor and the Crumbly Floor. It will activate
a Crumbly Floor if it lands on one, and can activate buttons and
switches that it passes.
* JavaScript API: FailLevel(message) can be called from a doodad to kill
the player character. The Anvil does this if it collides with the
player while it's been falling.
