Asset improvements (#1557)

This PR does the following:
- Add `selfHosted.js`, which is a great option for users that wish to
self host the assets. Works well for both self hosting from the public
folder or via a CDN.
- Updates the docs for assets. We now have a dedicated page for assets
where all the options are more clearly explained. I also removed the
assets explanation from the main docs as the unpkg option should work
out of the box and setting up the assets is no longer necessary.
- Cleaned up the `refresh-assets` script. We now use common `types.d.ts`
file to define our types. All the other options then reuse them.
- Pulled out the `formatAssetUrl` into it's own file. It's now static an
no longer generated.
- `urls.d.ts`, `import.d.ts`, and newly added `selfhosted.d.ts` are now
also no longer generated as we can import the types from `types.d.ts`.
- You can now pass a subset of `assetUrls` to `<Tldraw />` and it will
override the default option with the passed in overrides. This makes it
easy to only customizes certain assets (only change the draw font as an
example).

### Change Type

- [x] `patch` — Bug Fix

packages/assets/utils.d.ts

@@ -0,0 +1,2 @@
+import { AssetUrl, AssetUrlOptions } from './types'
+export function formatAssetUrl(assetUrl: AssetUrl, format: AssetUrlOptions): string