This adds a `isSnapId` function which accepts any value, and returns a
boolean if the input is a valid Snap ID or not.

This is extracted from #2634.

Like with websites, we want to support a custom network per Snap rather
than relying on the globally selected network. For Snaps, the permission
for the network to switch to is automatically granted.

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1. Copy the build files for `@metamask/selected-network-controller` from
[this PR](MetaMask/core#4602).
2. Copy the build files for `@metamask/snaps-controllers` and
`@metamask/snaps-execution-environments` from [this
PR](MetaMask/snaps#2634).
3. Run `test-snaps` for the PR above (`yarn start` in
`packages/test-snaps`).
4. Install the Ethereum Provider Example Snap.
5. Switch the chain ID using the `test-snaps` button for the Ethereum
Provider example.
6. The Snap should now use the new network, without any permission
requests.

Step 1 and 2 are no longer necessary once these PRs are merged and
released.

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