Support "dtx" vim key combination when "x" is immediately to the right of the cursor

[brianstrauch]

Closes #4358

The bug originates on this line: https://github.com/zed-industries/zed/blob/5db7e8f89ee4f5d9801a2eab88adc1db3fda0f46/crates/vim/src/motion.rs#L451

• When running "dtx" on "ˇabcx", the range to delete is 0 -> 2 ("abc")
• When running "dtx" on "abˇcx", the range to delete is 2 -> 2 ("c"), so new_point == point and the function incorrectly returns None and "c" is not deleted
• We need to disambiguate between the "not found" case and the "found immediately to the right" case
• This bug does not apply to the backwards case ("dTx")

Release Notes:

• Fixed "dtx" vim key combination when "x" is immediately to the right of the cursor (#4358)