add optional clearing of cursorword highlight on leave

[satorunooshie]

Summary

This PR adds an optional behavior to clear CursorWord highlights when
leaving a window or buffer, and refactors the highlight-clear logic into a
reusable function.

Motivation

When moving between windows or buffers, cursorword highlights can remain
visible in the window you just left, which can be visually noisy. Some users
prefer that highlights are cleared as soon as they leave a window, while
others prefer the current behavior, so this should be configurable.

Changes

• Add cursorword#clear() in autoload/cursorword.vim to:

  • Delete the current window's CursorWord match (if present).
  • Reset w:cursorword_match and w:cursorword_state.

• Refactor the timer-based cursorword#cursormoved() to call
  cursorword#clear() instead of duplicating the clear logic inline, keeping
  its behavior identical to before.

• Add a new global option g:cursorword_clear_on_leave:

  • When non-zero, cursorword#clear() is called on WinLeave,BufLeave.
  • When unset or zero, WinLeave,BufLeave does nothing and the original
    behavior is preserved.

• Keep the non-timer cursorword#cursormoved() implementation as
  call cursorword#matchadd() so there is no behavioral change when timers
  or g:cursorword_delay disable the timer path.

Usage

To enable clearing highlights when leaving a window or buffer:

let g:cursorword_clear_on_leave = 1

Without this setting, cursorword behaves exactly as before.