* Add the JavaScript system for Doodads to run their scripts in levels,
and wire initial OnCollide() handler support.
* CLI: Add a `doodad install-script` command to the doodad tool.
* Usage: `doodad install-script <index.js> <filename.doodad>`
* Add dev-assets folder for storing source files for the official
default doodads, sprites, levels, etc. and for now add a JavaScript
for the first test doodad.
Adds the `doodad` binary which will be a command line tool to work with
Doodads and Levels and assist with development.
The `doodad` binary has subcommands like git and the first command is
`convert` which converts between image files (PNG or BMP) and Doodle
drawing files (Level or Doodad). You can "screenshot" a level into a PNG
or you can initialize a new drawing from a PNG.
