Add support for relative paths in file and dir sigils

[kpbaks]

Description

I use file and dir sigils a lot as a way to check that I update all path references whenever I move a file or dir that some other file or tool in my repo refers to. I find it cumbersome to refer to the absolute path to the file relative to the workspace root. Since often I only care about the relative path to it from some other file.

I think it would be easier and more flexible to be able to refer to files using relative paths e.g. [file:./other_file.txt]