[1/3] initial highlighter shape/tool (#1401)

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!

![Kapture 2023-05-17 at 15 37
33](https://github.com/tldraw/tldraw/assets/1489520/982e78f4-6495-4a68-aa51-c8f7b5bcdd01)

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]

packages/editor/src/lib/utils/debug-flags.ts

@@ -11,6 +11,7 @@ export const featureFlags = {
 	// todo: remove this. it's not used, but we only have one feature flag and i
 	// wanted an example :(
 	peopleMenu: createFeatureFlag('peopleMenu'),
+	highlighterTool: createFeatureFlag('highlighterTool'),
 } satisfies Record<string, DebugFlag<boolean>>
 
 /** @internal */