This is the same code as the famous "Highlight all occurrences of selected word", originally authored by Nicholas Sha, except I added the ability to customize the colors, ignore commented text, and match case. See "Selection Highlighter++" under the Edit menu.
For those who aren't familiar with this... this extension makes it to where when you highlight a word, VS will highlight all occurrences of that same word in the text view. (Yes, I know VS already does this to an extent, but VS only does this with actual code symbols.) This does it with all text just like the way Notepad++ does it.