Add `shift shift` to open command palette (#13556)

I've add `shift shift` as a default keybinding to open command palette,
when using JetBrains keymap, along with the already existing
`cmd-shift-a`. This isn't quite right, as in JetBrains, `cmd-shift-a`
opens the actions modal, which would be our command palette, and `shift
shift` actually opens up a view for searching everything, commands,
actions, settings, etc - we do not have a unified modal for these
things, so I think this is the best thing we can do. Some users might
want to change this to be our file picker, but I think adding it as the
default at least puts it on their radar that they can use this type of
binding; they can change it if they want.

Release Notes:

- Added `shift shift` as a default binding to open the command palette
in the JetBrains keymap.