gh-38429: provide the fraction field of symmetric functions
We provide a naive implementation of the fraction field of symmetric
functions. In particular, this allows computations like
```
sage: s = SymmetricFunctions(QQ).s()
sage: s[1]/(1+s[1])
s[1]/(s[] + s[1])
```
URL: https://github.com/sagemath/sage/pull/38429
Reported by: Martin Rubey
Reviewer(s): Travis Scrimshaw
