Type normalisation

[andrewnicols]

This is part of a followup to #121. It:

• moves the type validation to its own Util class. In retrospect I should have done so whilst writing the Sniff but I was waiting until I wrote the next sniff that used it. However, moving it here allows better unit testing
• adds null, true, and false

It also moves the existing union type checks into the same TypeUtil class.
DNF + Intersection are separate followups.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.81%. Comparing base (24bc48a) to head (6027bd8).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #126      +/-   ##
============================================
- Coverage     97.81%   97.81%   -0.01%     
+ Complexity      827      825       -2     
============================================
  Files            35       36       +1     
  Lines          2426     2425       -1     
============================================
- Hits           2373     2372       -1     
  Misses           53       53              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.