[Merged by Bors] - feat(set_theory/cardinal): add add_le_omega
#9887
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c₁ + c₂ ≤ ω
toc₁ ≤ ω ∧ c₂ ≤ ω
;ω + ω
toω
;#s ≤ ω
tos.countable
;(s ∪ t).countable
and(insert a s).countable
.Motivated by lemmas from flypitch.