Symbolic dynamics: new PR

Stream: mathlib4

[Silvère Gangloff]

Hi all, I just opened a PR introducing some basic definitions in symbolic dynamics:
https://github.com/leanprover-community/mathlib4/pull/28546

It adds the full shift, cylinders, patterns, subshifts (via forbidden patterns),
and a first definition of entropy.

This is intended as a first step towards a small API for symbolic dynamics.
Feedback is very welcome!