Implement "a" and "A" commands

[valxntine]

A core keybinding in vim is the a and A commands, for appending from the current cursor position and appending at the end of the current line.

This work should also include tests reflecting the functionality of both functions of the a key.

[tauraamui]

@valxntine we should be able to take advantage of this from the V stdlib when the time comes: https://modules.vlang.io/strings.html#find_between_pair_u8