CLI #99 — Add an update check that notifies users of a newer CLI version

## Task
On startup, asynchronously check npm for a newer version of `@stellar-explain/cli` and print a notice if one is available. Must not block the command.

## Files to create
```
packages/cli/src/lib/updateCheck.ts
```

## Files to update
```
packages/cli/src/index.ts
```