Insert tab after current tab shortcut

[sheaf]

It would be nice to have a dedicated shortcut to open a tab next to the current one (e.g., ⌘+⌥+T on MacOS). Currently (I think) we can only override the default new tab behavior.

[Jon]

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[pewpew]

There are a few issues with `insertAfterCurrent`.

Clicking on "+ New Tab" still opens tab to the right (or below) of the current tab which is a weird UX. I'd expect it to open where I'm clicking on +.

I think all of this could be easily solved with a shortcut.