core: flatten assembly format class hierarchy

[alexarice]

Removes a few of the extra classes from the assembly format directives, instead replacing them with methods on the Directive class. This should make it simpler to implement custom directives in the future

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.26%. Comparing base (9accdb1) to head (86c281c).
Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3893   +/-   ##
=======================================
  Coverage   91.26%   91.26%           
=======================================
  Files         466      466           
  Lines       57949    57961   +12     
  Branches     5575     5567    -8     
=======================================
+ Hits        52886    52898   +12     
- Misses       3635     3639    +4     
+ Partials     1428     1424    -4     

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