Add singular files to search path instead of just directories and archives

[Charanor]

We should be able to add a singular file to the search path instead of an entire directory, like so:
fs.addToSearchPath("./config/graphics.ini", FileLocation.EXTERNAL);

This would help in cases where separating just a singular file into its own directory is ugly and will lead to bad folder structure for developers, just so that they can isolate a singular file.