allow theme subfolders

[t-fritsch]

allows custom themes to import files from subfolders inside the css/theme/source folder.

in css/theme/source/custom-theme.scss we can now do

@import `custom-theme/controls`;
@import `custom-theme/headings`;
...

This is usefull when creating custom themes with lot of features to split sass rules in different scss files.

Also, passing file parameter to sass.render() makes @import instructions work without the need to specify includePaths (that is a hack sometimes used when working only with the data parameter since sass doesn't know where the file is and then cannot do relative imports).

As stated in https://sass-lang.com/documentation/js-api/interfaces/legacystringoptions/#file :

If file and data are both passed, file is used as the path of the stylesheet for error reporting, but data is used as the contents of the stylesheet.

Since sass now knows where each file is, it can do relative @import without trouble.