This PR adds a `severity` argument so severity can be defined when
navigating through diagnostics. This allows keybinds like the following:

```json
{
  "] e": ["editor::GoToDiagnostic", { "severity": "error" }],
  "[ e": ["editor::GoToDiagnostic", { "severity": "error" }]
}
```

I've added test comments and a test. Let me know if there's anything
else you need!

Release Notes:

- Add `severity` argument to `editor::GoToDiagnostic`,
`editor::GoToPreviousDiagnostic`, `project_panel::SelectNextDiagnostic`
and `project_panel::SelectPrevDiagnostic` actions