This diff adds an initial version of the highlighter shape. At this
stage, it's a complete copy of the draw tool minus the following
features:
* Fills
* Stroke types
* Closed shapes
I've created a new shape util (a copy-paste of the draw one with stuff
renamed/deleted) but reused the state chart nodes for the draw shape.
Currently this new tool looks exactly like the draw tool, but that'll be
changing soon!
The UI here is extremely WIP. The highlighter tool is behind a feature
flag, but once enabled is accessible through the tool bar. There's a
first-draft highlighter icon (i didn't spend much time on this, it's not
super legible on non-retina displays yet imo), and the tool is bound to
the `i` key (any better suggestions? `h` is taken by the hand tool)
### The plan
1. initial highlighter shape/tool #1401 **>you are here<**
2. sandwich rendering for highlighter shapes #1418
3. shape styling - new colours and sizes, lightweight perfect freehand
changes
### Change Type
- [x] `minor` — New Feature
### Test Plan
(not yet)
### Release Notes
[internal only change layout ground work for highlighter]
