We saw an issue on stackblitz where the `global` constant was causing a
conflict (possibly because that environment was injecting its own
`global`). Either way, perhaps `global` is a bit of a risky name—this PR
renames `global` to `tldrawStateGlobal`.
### Change Type
- [x] `patch`
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
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Co-authored-by: David Sheldrick <d.j.sheldrick@gmail.com>