Reduce complexity in Selector._setSelectorText

[jaraco]

Reduces mccabe cyclomatic complexity from 88 to 7.

Ref #47

• Use contextlib.suppress
• Short circuit and reduce indentation.
• Extract _prepare_tokens
• Move the 'New' object into a dataclass.
• Moved append function into 'New' class.
• Moved productions into the 'New' class (and constants into their own class).
• Remove 'else' clauses; none of the 'if' clauses fall through.

[jaraco]

I'm going to ignore the diffcov failures. 9% of the code that was moved isn't covered by tests, which isn't great, but isn't the end of the world either, considering that the logic is meant to be unchanged.