Feature Request - Draggable Points

So far super impressed with this library!

## Problem
FL Chart currently doesn't support dragging data points, which limits its use
for interactive data editing applications (scheduling, profile editing, etc.)

## Proposed Solution
Add optional drag-and-drop functionality to LineChart that allows users to:
- Select data points by tapping
- Drag selected points to new positions
- Receive callbacks with updated point positions
- [Optional] Validate moves via callbacks (e.g., prevent out-of-bounds updates)

## Use Case
I've implemented this in a pretty proprietary fashion, including gesture detection, throttling, nearest point detection, and final position capture for a production app and might be able to contribute it back.

Would it be helpful if I attempt to adapt it to a PR?

Start by locating the LineChart implementation and its existing gesture or point-selection behavior. Define how tapping, dragging, updated-position callbacks, and optional move validation should work, then verify that the requested interactions are covered by tests or an interactive example.

Written by the indexing model from the issue text.