Add support for text files (markdown)

[Wombosvideo]

Plain markdown (.md) files are currently not supported, even though they're just plain text with some formatting characters.

[luleyleo]

Any plain text file should be searched, and markdown files certainly work. Where does the search fail for you?

[Wombosvideo]

I just performed a multitude of searches in the last three hours and markdown files do in fact work, but not all the time. If I get to it, I will try to identify which files or searches it fails for.

[Wombosvideo]

So I wasn't able to identify how search works and doesn't work. I can still notice differences between VSCode's search and this search but I don't think it's worth exploring any further if the majority of users are happy with this search algorithm.