feat(measure_theory/measure/haar): Add the Steinhaus Theorem (#12932)
feat(measure_theory/measure/haar): Add the Steinhaus Theorem (#12932)

This PR proves the [Steinhaus Theorem](https://en.wikipedia.org/wiki/Steinhaus_theorem) in any locally compact group with a Haar measure. 



Co-authored-by: YaelDillies <aef64c2b10aaee4bd1b6422c57104205a3c50e06@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib