✨ Column selection

### Description

We need to implement column selection (also known as block selection or rectangular selection) in CodeEdit, allowing users to select text in a columnar fashion. This feature is particularly useful for editing structured text, such as code, where the same changes need to be applied to multiple lines simultaneously.

**Triggering Column Selection:**
- Users should be able to initiate column selection by either
- holding down the `⌥ Option` key and dragging the mouse or
- using the keyboard with `⇧ Shift` + `⌃ Control` + `▲ up`/`▼ down` arrow keys

**Visual Indication:**
- There should be a clear visual indication that column selection mode is active by changing the cursor to crosshairs

**Editing Behavior:**
- When text is typed while in column selection mode, it should replace the selected text in all lines within the selection.
- Deleting should remove the selected block of text across all lines.
- Navigating and selection with the arrow keys should apply to each cursor as expected, including with modifier keys held

**Clipboard Support:**
- Copying should retain the multi-line selection.

### Additional Context

This feature is common in many text editors and IDEs, such as Visual Studio Code, Sublime Text, and JetBrains IDEs, and can significantly enhance productivity, especially when working with large codebases.

### Possible Challenges

- Ensuring that column selection works seamlessly with all supported languages and file types.
- Managing edge cases, such as selections that start or end in the middle of a tab character or mixed tab/space indentations.

### References

- [VSCode's Column Selection Mode](https://code.visualstudio.com/docs/editor/codebasics#_column-box-selection)
- [Sublime Text Column Selection](https://www.sublimetext.com/docs/column_selection.html)

### Tasks
1. Implement the core column selection functionality.
2. Change cursor when ⌥ is held and over editor to indicate column selection mode.
3. Ensure clipboard operations work with column selections.
4. Test for compatibility with other editor features and languages.
5. Document the feature in the CodeEdit user guide.

### Screenshots

https://github.com/user-attachments/assets/55c15e53-d341-417e-81d6-9aaccbe1fc9f