Add feature type unknown definition

[HarelM]

This add the unknown type definition for feature - "0".
I've seen that the definition here does not conform to the spec.
I'm trying to see if I can change it in all the relevant places...

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (edeaee3) 78.02% compared to head (45d3069) 78.02%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #365   +/-   ##
=======================================
  Coverage   78.02%   78.02%           
=======================================
  Files         101      101           
  Lines        4142     4142           
  Branches     1184     1184           
=======================================
  Hits         3232     3232           
  Misses        910      910           

Files	Coverage Δ
src/expression/index.ts	70.10% <ø> (ø)

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