Begin implementation of null

[mtfishman]

This gets some of the interface infrastructure in place for left_null/right_null, including throwing errors instead of silently giving strange results.

My plan is to merge this and work on the actual implementation in a future PR. Both QR and SVD backends will be a little subtle to implement.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 74.15%. Comparing base (2578245) to head (013e0c2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/factorizations/orthnull.jl	0.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   74.94%   74.15%   -0.80%     
==========================================
  Files          34       34              
  Lines        1688     1706      +18     
==========================================
  Hits         1265     1265              
- Misses        423      441      +18     

Flag	Coverage Δ
docs	6.68% <0.00%> (-0.08%)	⬇️

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