chore: cleanup z-indices so that they're all clearly listed (#3855)

minor thing that was bugging me. we had a bunch of z-index vars that
were "unregistered". this brings them back into a consistent place so we
can see from a bird's eye view easily the stacking order of everything.

also, a drive-by fix to not duplicate `--tl-text-outline` inside of
`DefaultCanvas`

### Change Type

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- [x] `sdk` — Changes the tldraw SDK
- [ ] `dotcom` — Changes the tldraw.com web app
- [ ] `docs` — Changes to the documentation, examples, or templates.
- [ ] `vs code` — Changes to the vscode plugin
- [ ] `internal` — Does not affect user-facing stuff

<!-- ❗ Please select a 'Type' label ❗️ -->

- [ ] `bugfix` — Bug fix
- [ ] `feature` — New feature
- [ ] `improvement` — Improving existing features
- [x] `chore` — Updating dependencies, other boring stuff
- [ ] `galaxy brain` — Architectural changes
- [ ] `tests` — Changes to any test code
- [ ] `tools` — Changes to infrastructure, CI, internal scripts,
debugging tools, etc.
- [ ] `dunno` — I don't know

packages/editor/src/lib/components/default-components/DefaultCanvas.tsx

@@ -69,11 +69,7 @@ export function DefaultCanvas({ className }: TLCanvasComponentProps) {
 				const lodDisableTextOutline = z < editor.options.textShadowLod
 				container.style.setProperty(
 					'--tl-text-outline',
-					lodDisableTextOutline
-						? 'none'
-						: `0 var(--b) 0 var(--color-background), 0 var(--a) 0 var(--color-background),
-				var(--b) var(--b) 0 var(--color-background), var(--a) var(--b) 0 var(--color-background),
-				var(--a) var(--a) 0 var(--color-background), var(--b) var(--a) 0 var(--color-background)`
+					lodDisableTextOutline ? 'none' : `var(--tl-text-outline-reference)`
 				)
 				rMemoizedStuff.current.lodDisableTextOutline = lodDisableTextOutline
 			}