feat(model_theory/terms_and_formulas): Notation for terms and formulas from Flypitch (#12630)
feat(model_theory/terms_and_formulas): Notation for terms and formulas from Flypitch (#12630)

Introduces some notation, localized to `first_order`, to make typing explicit terms and formulas easier.



Co-authored-by: Aaron Anderson <6a708ae528ec3e8bbcd87cbd53c0d4f4bab63d64@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib