better invalid unit error handling

[scytacki]

• the UnitManager now doesn't crash when initializeMath is called and there is an invalid unit
• the _ char is explicitly allowed to be a unit character
• invalid units are not added to the unit manager
• updated custom-math test so each test is isolated with its own instance of the units manager.
• removed what seemed like an unnecessary explicit "addUnit" call in variable.test.ts
• grouped invalid unit tests in variable.test.ts
• added tests for invalid characters of various types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.05%. Comparing base (f8c2080) to head (b4d827b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   72.03%   72.05%   +0.02%     
==========================================
  Files          47       47              
  Lines        1280     1281       +1     
  Branches      295      295              
==========================================
+ Hits          922      923       +1     
  Misses        343      343              
  Partials       15       15              

Flag	Coverage Δ
jest	72.05% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[bgoldowsky]

LGTM 👍