Stress `filename` option as *a must* for most programmatic `less.render` use-cases.

[seven-phases-max]

See #2342, #2583. (relative imports, ulrs, data-uris as well as source-maps and similar compiled-less-file-depended things just won't work properly w/o this option).

[tustin2121]

While you're at it, how about specifying what all can be put into the options object when doing a programmatic less.render. The only thing the "docs" say, at time of writing, is that options are optional, and nothing about what options there even are! And the source code is too obtuse to try and find where the options are being used...

[seven-phases-max]

@tustin2121 Probably, but this is beyond this ticket scope. Create a new one if necessary.