Draft: Introduce new special variable, $FILEDIR

Merged Kevin Hovsäter requested to merge hovsater/dte:filedir-variable into master

Expands to the directory of the current buffer or an empty string in case the buffer is not yet associated with a file.

This can be useful if you for instance want to create files relative to the current file. For instance, this allow you to do :open $FILEDIR/../parent_file.c as an example.