feat: support reverting only selected text in Revert Tree String to Text

[nisedo]

Summary

• Allow Revert Tree String to Text command to revert only the selected text when a selection exists
• Preserve the original behavior of reverting the entire file when no text is selected

Motivation

Previously, the "Revert Tree String to Text" command would always revert the entire document, regardless of whether text was selected. This made it impossible to selectively revert portions of a tree file.

With this change, users can now:

1. Select specific tree sections and revert only those portions
2. Use the command without selection to revert the entire file (original behavior preserved)

Changes

• Modified extension.asciiTreeGeneratorRevertToText command in src/extension.ts to check for active selection
• Updated CHANGELOG.md with the change under [Unreleased]

Test plan

• All existing tests pass (yarn test)
• Linter passes (yarn lint)
• Code is properly formatted (yarn prettify)
• Manual testing:
  • Open a .tree file with formatted tree content
  • Select a portion of the tree and run "Revert Tree String to Text" → only selected text should be reverted
  • Run "Revert Tree String to Text" without selection → entire file should be reverted