Incorporate signia as @tldraw/state (#1620)

It tried to get out but we're dragging it back in. 

This PR brings [signia](https://github.com/tldraw/signia) back into
tldraw as @tldraw/state.

### Change Type

- [x] major

---------

Co-authored-by: David Sheldrick <d.j.sheldrick@gmail.com>

packages/state/src/lib/core/isSignal.ts

@@ -0,0 +1,11 @@
+import { _Atom } from './Atom'
+import { _Computed } from './Computed'
+import { Signal } from './types'
+
+/**
+ * Returns true if the given value is a signal (either an Atom or a Computed).
+ * @public
+ */
+export function isSignal(value: any): value is Signal<any> {
+	return value instanceof _Atom || value instanceof _Computed
+}