Changes default keymaps to more closely match the behavior of VSCode.

New Zed behavior:
`cmd-k w` / `ctrl-k w` -- Closes all buffers in the current pane
`cmd-k cmd-w` / `ctrl-k ctrl-w` -- Closes all buffers in all panes

VScode:
`cmd-k cmd-w` is workbench.action.closeAllEditors (close all buffers in
all splits)
`cmd-k w` is workbench.action.closeEditorsInGroup (close all buffers in
current split)

Both leave pinned tabs untouched.

Release Notes:

- Improved keybindings for close all tabs to better match VSCode
behavior