### What does this PR do?
- Adds default keybindings `gt` for navigating to the next tab and `gT`
for navigating to the previous tab in markdown viewer mode

### Why do we need this change?
- While previewing markdown files, the default vim bindings (`gt` and
`gT`) do not work for navigating between tabs. These bindings work
everywhere else, which provides a non-consistent experience for the
user.

### How do we do this change?
- Update the vim mode bindings to explicitly add handling for this mode

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>