Implement changes necessary to format output

* Universally use parse() instead of __toString
* Introduce OutputFormat class whose instance can be passed to parse()
* Prepare release for 6.0.0
Make Format mandatory when calling render()

use __toString() for the default format except in \Sabberworm\CSS\CSSList\Document where null is still allowed

This removes all those null checks.
Initial output formatting options

catching output exceptions not yet implemented
Fix unit tests for changed default output.
Ignore Exceptions during rule set and list render

if output format has `setIgnoreExceptions(true)` or `beLenient()`