[vim] Automatically set RUNEWIDTH_EASTASIAN=1 when &ambiwidth == double

CHANGELOG.md

@@ -92,6 +92,7 @@ CHANGELOG
   for `--border-label`)
 - Better support for (Windows) terminals where each box-drawing character
   takes 2 columns. Set `RUNEWIDTH_EASTASIAN` environment variable to `1`.
+    - On Vim, the variable will be automatically set if `&ambiwidth` is `double`
 - Behavior changes
     - fzf will always execute the preview command if the command template
       contains `{q}` even when it's empty. If you prefer the old behavior,