Explicitly declare root instantiables

[dfed]

Finding the @Instantiable-decorated types that are roots of a dependency tree automatically is a bit magical, but also comes with a concrete downside: when writing a brand new type that only has @Instantiated dependencies, SafeDI complains during builds until that new type is @Instantiated by another @Instantiable-decorated type.

This case is difficult for newcomers to SafeDI to debug, and also annoys veteran SafeDI consumers.

This PR represents a breaking change from the 0.9.* release, and is intended to land prior to a 1.0.0 release. This PR will land after a 0.9.1 release is created.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (15927f1) to head (1028470).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #126   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files          33       33           
  Lines        3453     3496   +43     
=======================================
+ Hits         3442     3485   +43     
  Misses         11       11           

Files with missing lines	Coverage Δ
...eDICore/Extensions/AttributeSyntaxExtensions.swift	100.00% <100.00%> (ø)
...afeDICore/Generators/DependencyTreeGenerator.swift	100.00% <100.00%> (ø)
Sources/SafeDICore/Models/Instantiable.swift	100.00% <100.00%> (ø)
Sources/SafeDICore/Models/Property.swift	100.00% <100.00%> (ø)
...rces/SafeDICore/Visitors/InstantiableVisitor.swift	100.00% <100.00%> (ø)
...ources/SafeDIMacros/Macros/InstantiableMacro.swift	100.00% <100.00%> (ø)
Sources/SafeDITool/SafeDITool.swift	98.85% <100.00%> (+<0.01%)	⬆️

[NickEntin]

Documentation and tests LGTM