Fix double-click and triple-click selection behavior

[nicholasirwin]

Description

This PR changes double-click and triple-click selection in Textual so they select words and paragraphs instead of the whole widget.

• Double-click now selects the word at the click position (including URLs and words with apostrophes/hyphens).
• Triple-click now selects the paragraph at the click position.
• Both fall back to the previous behavior (select widget / select container) when the click cannot be mapped to text content.

Changes

• Added textual._word_segmentation module with an optional regex dependency and a re fallback.
• Added textual._text_selection helpers for word/paragraph segmentation with CRLF-aware offset math.
• Added Widget.get_selectable_text(), Widget.get_word_selection(), Widget.get_paragraph_selection(), Widget.text_select_word(), and Widget.text_select_paragraph().
• Updated Widget._on_click() to use the new selection methods.
• Refactored Widget.get_selection() to share text extraction logic with get_selectable_text().
• Fixed stale _select_state by clearing it when mutating Screen.selections directly.
• Preserved the container-select fallback for triple-click on non-text widgets.

Test plan

pytest tests/test_selection.py tests/test_text_selection.py

Fixes #6633