Add `error` deprecation level

Follow up to #2107 from proposal https://gist.github.com/timotheeguerin/f525f392a5d39e3d3d92260167e62c3e

**This is not needed at the time is created to track intrest.**

A 2nd optional parameter to the deprecation directive could be added later which lets you upgrade the severity to an `error`

Advantage of this is you can use that to show that something was removed by give it a better error than it just being gone to users.

You could also imaging a deprecation cycle that increase the severity.

1. warning: will be removed soon
2. error: has been removed

```tsp
#deprecate "This has been removed in version 2.0, use X instead." "error"
```

What should be emitted if referencing a type with the following deprecated annotations

| Case | Default | `--check-deprecation` |
| ------------------------------------ | ------------ | --------------------- |
| `#deprecated "Default"` | diag warning | diag warning |
| `#deprecated "As warning" "warning"` | diag warning | diag warning |
| `#deprecated "As error" "error"` | diag error | diag error |