New `suggestion` severity

With the introduction of the built-in linter system and linter rules only able to be `warning` it would be good to have a lower severity

I think the severities can be seen this way:
- `error`: This is breaking we are not able to finish the compilation/emitting because of that error
- `warning`: Doesn't prevent development but you need to resolve before merging (e.g. something that would be invalid in a generated language)
- `suggestion`: This would be ideal if it was fixed but will not be major problem if not. (e.g. casing issue)

## Uses cases

List of cases were it could make sense to use hint
- unused using, import, template parameter, etc.