feat: Allow nav_item to be opened in a new tab/window #1932

The PR fulfills these requirements: (check all the apply)

[x] It's submitted to the main branch.
[x] When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
[x] When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
[x] If changes were made to ui folder, unit tests (make test) still pass.
[x] New/updated tests are included

Closes #1932

h2oai / wave