Add selectable markdown context menu builder

[momadvisor]

Summary

• Adds a contextMenuBuilder parameter to Markdown and MarkdownBody.
• Passes the builder through to the internal SelectableText.rich widgets when selectable is enabled.
• Preserves the existing default Flutter selection toolbar behavior when the parameter is omitted.
• Allows callers to set contextMenuBuilder: null to suppress the selectable text context menu.
• Updates selectable markdown selection callbacks to pass TextSpan.toPlainText(), so callers receive the full selectable plain text even when markdown produces rich/nested spans.

Motivation

Apps embedding selectable markdown may already provide their own surrounding context menu. Without a way to customize or suppress the internal SelectableText context menu, right-click/long-touch behavior can conflict with the app-level menu.

Also, consumers that implement their own selection-related UI need reliable plain text from onSelectionChanged. For rich markdown spans, using only TextSpan.text can be null or incomplete, while toPlainText() reflects the rendered selectable text.

Tests

• flutter test test/context_menu_builder_test.dart
• flutter analyze