Crouzeix's conjecture

### What is the conjecture
A conjecture in matrix analysis.

Source: https://en.wikipedia.org/wiki/Crouzeix%27s_conjecture

### Prerequisites needed
Would need API for $W(A)$, the field of values of the (complex) matrix A.

### [AMS categories](https://github.com/google-deepmind/formal-conjectures/labels?q=ams-)

* ams-15

### Choose either option
- [ ] I plan on adding this conjecture to the repository
- [x] This issue is up for grabs: I would like to see this conjecture added by somebody else

Start with the linked Wikipedia description of Crouzeix's conjecture and review the repository's existing formalized conjectures in matrix analysis. Determine the required API for W(A), the field of values of a complex matrix, then formalize and add the conjecture to the repository; done means the statement is accepted by Lean.

Written by the indexing model from the issue text.