Redmine

Add Tab / Shift+Tab support for indenting and unindenting list items in the wiki textarea when CommonMark formatting is used.

- Tab indents the current list item.
- Bullet list items (`-`, `*`, `+`) are indented by 2 spaces.
- Ordered list items (`1.`, `2)`, …) are indented by 4 spaces.
- Shift+Tab unindents the current list item by removing leading spaces.
- If the current indentation is smaller than the step size, only the existing spaces are removed.
- If there is no indentation, nothing is changed.

On non-list lines, Tab keeps its normal browser behavior and moves focus.

This feature applies only to CommonMark formatting. Textile uses repeated markers such as `*`, `**`, and `***` for nested lists, so indentation shortcuts are not needed.