Code fix to auto-import custom types / variables functions

If I know the name of a custom type that I've defined elsewhere (e.g. `ContainerImage`), when I type the following I'm going to get an error because `ContainerImage` hasn't been imported:

```bicep
param image ContainerImage
```

It would be really nice to get a code fix on `ContainerImage` to automatically add the import for me:

```bicep
import { ContainerImage } from '../shared/types.bicep'

param image ContainerImage
```

Start with the unresolved `ContainerImage` example and the import syntax shown for `../shared/types.bicep`. Trace how the missing custom type is surfaced and how code fixes are offered; done when selecting `ContainerImage` can add the defining import while preserving `param image ContainerImage`.

Written by the indexing model from the issue text.