cmp: update keymaps

[MikaelFangel]

Unfortunately the current keymaps interfere with your writing flow because copilot eagerly tries to complete a line even if it only contains whitespace. This makes it hard to write languages that are whitespace-sensitive e.g. Python and F#.

Changes

• This updates the keycap for completion and makes it so autocompletion won't trigger on lines with only whitespace
• Shadows completion text

Keymap	Functionality
Tab	Autocomplete if at least one non-whitespace is present
S-Tab	Close completion menu
Up	Select prev completion
Down	Select next completion

CC @PhilipCramer @Greve2001 - For opinions and testing.