[TASK] Make RuleSet concrete

[JakeQZ]

... adding internal render method.

Precursor to #1194.

[coveralls]

coverage: 59.794% (+0.04%) from 59.751%
when pulling 6fdb2e3 on task/ruleset-concrete
into 8c4a77e on main.

[oliverklee]

Looks generally good.

I'd suggest moving the tests for render() to functional tests, though, as they also test the effects of OutputFormat/OutputFormatter.

[JakeQZ]

the tests ... also test the effects of OutputFormat/OutputFormatter.

Only insofar as the expected CSS matches that rendered with the default OutputFormat - there aren't any tests for varying the spacing.

[JakeQZ]

the tests ... also test the effects of OutputFormat/OutputFormatter.

Only insofar as the expected CSS matches that rendered with the default OutputFormat - there aren't any tests for varying the spacing.

I note the equivalent tests for other classes are in the Functional department, and also test with pretty and compact formats. So this should probably test those too...