Backspace onLongPress callback added

[vedasjad]

Summary

This pull request introduces an improvement to the behavior of the backspace button. The original implementation required multiple taps to delete multiple characters at a time, and this update ensures continuous deletion when the user presses and holds the backspace button. Also deletes word by word after 3 seconds have passed since backspace is pressed.

Changes Made

1. GestureDetector Update: Wrapped the IconButton in GestureDetector and added a timer. Now, continuous backspace deletion occurs while the user's finger is on the backspace button.

Testing

• Verified that the backspace button triggers continuous deletion when the user presses and holds the button.
• Ensured that deletion stops when the user lifts their finger from the backspace button.
• The UI is exactly similar as before.

Here's the example video :
https://github.com/Fintasys/emoji_picker_flutter/assets/96325125/c5177e47-e47e-4ef5-82a7-29d149b144a3

[Fintasys]

@vedasjad Thank you for your great contribution! The implementation overall looks good IMO. The CI is still failing due to

info • The line length exceeds the 80-character limit • lib/src/default_emoji_picker_view.dart:231:81 • lines_longer_than_80_chars

If you could fix this issue I'm think we can merge it 👍

[vedasjad]

I've reduced the line length as instructed.
Please, see if now it passes all the tests.