[fix] Cleanup text measures (#3169)

This PR cleans up text measurement divs, which could pile up during HMR.

### Change Type

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

- [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 ❗️ -->

- [x] `bugfix` — Bug fix
- [ ] `feature` — New feature
- [ ] `improvement` — Improving existing features
- [ ] `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


### Release Notes

- Fixed a bug that could cause multiple text measurement divs in
development mode.

packages/editor/src/lib/editor/managers/TextManager.ts

@@ -40,12 +40,19 @@ export class TextManager {
 	baseElm: HTMLDivElement
 
 	constructor(public editor: Editor) {
+		const container = this.editor.getContainer()
+
+		// Remove any existing text measure element that
+		// is a descendant of this editor's container
+		container.querySelector('#tldraw_text_measure')?.remove()
+
 		const elm = document.createElement('div')
 		elm.id = `tldraw_text_measure`
 		elm.classList.add('tl-text')
 		elm.classList.add('tl-text-measure')
 		elm.tabIndex = -1
-		this.editor.getContainer().appendChild(elm)
+		container.appendChild(elm)
+
 		this.baseElm = elm
 	}