Defaults file does not accept ${.} for custom writer/readers

[svenboulanger]

When using a defaults file, it is currently impossible to use the ${.} for the path of the defaults file in the to and from properties. This is usually fine, except when trying to use a custom writer or reader where the filepath needs to be specified.

This defaults file fails:

from: commonmark_x
to: ${.}/customwriter.docx.lua

Version is pandoc 3.1.2. I propose to have the to and from fields to be treated as filenames (or maybe only if they aren't recognized as a "standard" input/output format).