Add two simple perf helpers. (#3399)

Can be useful for ad-hoc measure of performance. One is a method
decorator, which can be use on methods like so:

```typescript
  @measureDuration
  someLongRunningProccess() {
  // ....
  }
```

And the other offer more granular control. It also returns what the
callback returns, so it can be use in assignments / return statements.

```typescript
return measureCbDuration('sorting took', () =>  renderingShapes.sort(sortById))
```


### Change Type

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

- [ ] `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
- [x] `internal` — Does not affect user-facing stuff

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

- [ ] `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
- [x] `tools` — Changes to infrastructure, CI, internal scripts,
debugging tools, etc.
- [ ] `dunno` — I don't know

packages/utils/src/lib/perf.ts

@@ -0,0 +1,22 @@
+/** @internal */
+export function measureCbDuration(name: string, cb: () => any) {
+	const now = performance.now()
+	const result = cb()
+	// eslint-disable-next-line no-console
+	console.log(`${name} took`, performance.now() - now, 'ms')
+	return result
+}
+
+/** @internal */
+export function measureDuration(_target: any, propertyKey: string, descriptor: PropertyDescriptor) {
+	const originalMethod = descriptor.value
+	descriptor.value = function (...args: any[]) {
+		const start = performance.now()
+		const result = originalMethod.apply(this, args)
+		const end = performance.now()
+		// eslint-disable-next-line no-console
+		console.log(`${propertyKey} took ${end - start}ms `)
+		return result
+	}
+	return descriptor
+}