gh-38730: Expose Coxeter and dual Coxeter numbers in RootSystem class
Makes the Coxeter number and dual Coxeter number, which are properties
of a root system, available in the RootSystem class.
The underlying functions already exist in the CartanType, so this is a
matter of exposing them in the RootSytem class.
### :memo: Checklist
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- [X ] The title is concise and informative.
- [X ] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ X] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
### :hourglass: Dependencies
URL: https://github.com/sagemath/sage/pull/38730
Reported by: Skip G
Reviewer(s): Skip G, Travis Scrimshaw
