change-(with-)nth: apply value after current one

- Find current value in bound list and apply next valid,
  non-current one
- Keep rotating bound list as a position hint, so repeated
  values in the list still reach every position
- Treat empty --nth and '..' as the same value
- Ignore invalid nth values, also in transform-nth, as with-nth
  does. Previously, they restored default --nth
- Previously, first value of rotated list was applied blindly
    - Key press spent on value already applied, e.g. first value
      equal to initial --nth
    - Stuck on change-nth printed by transform (new action on
      each run), as it always applied first value
- Add nextValue helper for reuse by change-layout

CHANGELOG.md

@@ -9,6 +9,19 @@ CHANGELOG
     - Only `change-nth` and `change-with-nth` rotate through `|`-separated values
     - Fixed the bound command being replaced with the output. e.g. With `transform-nth(echo '1|2')`, the next key press ran `2|1` as a shell command.
     - A `--with-nth` template in the output can now contain `|`. e.g. `{2}|{1}` was cut to `{2}`.
+- An invalid value for `change-nth` and `transform-nth` no longer restores the default `--nth`. It is ignored, as with `change-with-nth` and `transform-with-nth`.
+- `change-nth` and `change-with-nth` now apply the value after the current one in the `|`-separated list
+    - A value equal to the current one is skipped, e.g. the first value when it is the initial `--nth`
+    - Invalid values in the list are skipped
+    - The rotation follows the current value even when another binding changed it, or when the action is printed by `transform` (unless the list contains the same value more than once)
+    - You can list the values in their natural order, starting with the initial one
+      ```sh
+      # Previously, you would have to write:
+      ls -al | fzf --with-nth .. --bind 'space:change-with-nth:1|2|..'
+
+      # Now you can write:
+      ls -al | fzf --with-nth .. --bind 'space:change-with-nth:..|1|2'
+      ```
 - nushell
     - The completion integration now requires nushell 0.116.1 or above, where an interactive completer can take the terminal through the `@interactive` attribute (#4934) (@sim590)