feat(textarea): expose MoveToBegin and MoveToEnd methods

[aymanbagabas]

This commit exposes the MoveToBegin and MoveToEnd methods in the textarea package, allowing users to programmatically move the cursor to the beginning or end of the input text.

[Copilot]

Pull Request Overview

This PR exposes two new public methods (MoveToBegin and MoveToEnd) on the Model to allow programmatic cursor movement, and updates internal calls to use these methods.

• Renames moveToBegin → MoveToBegin and moveToEnd → MoveToEnd and updates their comments.
• Updates all internal invocations in Update to use the new exported methods.

Comments suppressed due to low confidence (2)

textarea/textarea.go:986

• There aren’t any unit tests covering the newly exported MoveToBegin and MoveToEnd methods. Adding tests will ensure correct behavior and prevent regressions.

// MoveToBegin moves the cursor to the beginning of the input.

textarea/textarea.go:994

• If m.value can ever be empty, len(m.value)-1 will underflow, setting m.row to -1. Consider guarding against an empty value slice before setting the row.

	m.row = len(m.value) - 1