feat(number_theory/number_field): add definitions and results about embeddings  (#14749)
feat(number_theory/number_field): add definitions and results about embeddings  (#14749)

We consider the embeddings of a number field into an algebraic closed field (of char. 0) and prove some results about those. 

We also prove the ```number_field``` instance  for ```adjoint_root``` of an irreducible polynomial of `ℚ[X]`. 

Co-authored-by: Alex J. Best <alex.j.best@gmail.com>

From flt-regular



Co-authored-by: Xavier-François Roblot <52e65f2e4a84c0b65809bd9cb2f5514df5baca79@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib