add support for motion repetition

this PR adds the following support

in NORMAL mode, enter a number n and then press config.keybindings.navigation.down to go down n rows
in NORMAL mode, enter a number n and then press config.keybindings.navigation.up to go up n rows
in NORMAL mode, enter a number n and then press g to jump to line number n
in NORMAL mode, press control + d to jump half a page down (half page size is hardcoded for now)
in NORMAL mode, press control + u to jump half a page up (half page size is hardcoded for now)
in NORMAL mode, press g to jump to the top
in NORMAL mode, press G to jump to the bottom

amtoine / nu_plugin_explore