feat(input): add clear method

[TomJGooding]

Closes #3428 by adding a Input.clear method.

Example usage:

from textual.app import App, ComposeResult
from textual.widgets import Input


class InputApp(App):
    def compose(self) -> ComposeResult:
        yield Input(placeholder="Enter some text and press enter")

    def on_input_submitted(self, event: Input.Submitted) -> None:
        self.notify(f"You entered '{event.value}'")
        event.input.clear()


if __name__ == "__main__":
    app = InputApp()
    app.run()

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[willmcgugan]

Thanks, Tom.