feat: Add `{value_length}`, `{max_length}`, and `{symbols_left}` placeholders to `TextField.counter_text`

[ndonkoHenri]

Closes #1534

Test Code

import flet as ft

def main(page: ft.Page):
    page.add(
        ft.TextField(
            hint_text="Custom counter text",
            counter_text="{value_length} chars / {max_length} max chars / {symbols_left} symbols left",
            max_length=10,
        ),
        ft.TextField(
            hint_text="Counter text isn't shown by default anymore",
            max_length=20,
        ),
    )

ft.app(main)

Summary by Sourcery

Add new placeholders to TextField.counter_text for dynamic character count display, enhancing the user interface with real-time feedback on text input length.

New Features:

• Introduce placeholders {value_length}, {max_length}, and {symbols_left} in TextField.counter_text to dynamically display character count information.

Enhancements:

• Modify buildInputDecoration to support dynamic counter text with new placeholders.