Fixes #9089.

On linux, pressing shift and tab together can potentially produce
`ISO_Left_Tab`. This PR maps this key to "tab" with the shift modifier,
similar to `SHIFT_TAB_KEY` in gpui::platform::mac::events.

Note: The [default linux
keymaps](https://github.com/zed-industries/zed/blob/main/assets/keymaps/default-linux.json)
have shift-tab mapped to editor::TabPrev and ctrl-[ mapped to
editor::Outdent. Both actions appear to have the same effect.

Release Notes:

- Support shift-tab on linux (#9089).