Enable autoconvert in GssResourceGenerator.

This is still guarded by a flag and needs to be turned on by each app.

fixes #108
I'm wondering if you have to pass the original file here... The only reason to pass the file is to be able to display the name of the file with the location info (source and column) when an error occurs.
The conversion will remove comments, extra new lines,  reorganize several at-rules (`@external` and constant will be gathered at the beginning of the converted file), so the information about the source code (line and column) will not be accurate at all and won't match the line in the original file. That could be confusing.