chore!: Checks/validation on t.either

[j03-dev]

Emit a warning or an error when a variant is a subtype of another one.

Migration notes

BREAKING CHANGE: Previously valid typegraph might fail validation.
You will need to fix your types to add some consistency in t.either/t.union types.

• The change comes with new or modified tests
• Hard-to-understand functions have explanatory comments
• End-user documentation is updated to reflect the change

[linear]

MET-674 (graph) Checks/validation on `t.either`

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.01%. Comparing base (bbef981) to head (4fdfaa7).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #868      +/-   ##
==========================================
- Coverage   77.09%   77.01%   -0.08%     
==========================================
  Files         149      149              
  Lines       17849    17867      +18     
  Branches     1730     1737       +7     
==========================================
  Hits        13761    13761              
- Misses       4065     4083      +18     
  Partials       23       23              

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[Natoandro]

Very nice 👍
A few comments...

[Natoandro]

Can you also add a validation for TypeNode::Union which should fail validation if we have two variants with the same type.

[Natoandro]

The metagen test is failing...