Implements relative line jumping, a common feature i missed from
Vim/Neovim where you can jump to lines relative to your current cursor
position.

- New `{"relative": true}` flag for `go_to_line::Toggle` action 
- Supports both `-5` (5 lines up) and `b5` (backward 5 lines) syntax
- Full lines only (no column support in relative mode, wasnt sure if
that would be necessary)
- Unbound by default - users can add e.g. `"ctrl-j":
["go_to_line::Toggle", {"relative": true}]` to their keymap

Example usage:
- `5` → jump 5 lines down
- `-3` or `b3` → jump 3 lines up  
- `0` → stay on current line


[Screencast_20260115_191312.webm](https://github.com/user-attachments/assets/395c0e7b-8ac1-48c8-a39e-5ade4f9206ec)

Release Notes:

- Added relative line jump support to go-to-line action via
`+/-/f/b/F/B`