Add feature to check the diff between a snipped and selected text

[thgcode]

1. Save a snippet on one of the slot.
2. Select any text.
3. Press NVDA+CONTROL+SHIFT+D to view the diff between the snippet and selected text.

Or:

1. Select one of the saved snippets.
2. Select any text.
3. Press NVDA+CONTROL+SHIFT+D to view the diff between the snippet and selected text.