Rename till commands

[vwkd]

The till commands seem to be lacking the direction or the "find" in the name. Renaming

• find_till_char (t) to find_till_next_char,
• extend_till_char (t) to extend_till_next_char,
• till_prev_char (T) to find_till_prev_char,

would make them consistent with find_next_char (f) and find_prev_char (F), as well as the movement commands (move|extend)_(next|prev)_word_(start|end) (w, b, e, -).

This would be a breaking change for anyone remapping them to custom keybindings. To mitigate this, the old commands could be kept as aliases for a transitioning period.