Vim mode currently supports `gt` (go to next tab) and `gT` (go to
previous tab) but not with count. Implement the expected behavior as
defined by vim:

- `<count>gt` moves to tab `<count>`
- `<count>gT` moves to previous tab `<count>` times (with wraparound)

Release Notes:

- Improved vim `gt` and `gT` to support count, e.g. `5gt` - go to tab 5,
`8gT` - go to 8th previous tab with wraparound.