Handle Mul instances in vector.cross

[rho-novatron]

References to other Issues or PRs

Fixes #23749

Brief description of what is fixed or changed

Adds support for Mul instances as arguments to vector.cross.

Other comments

This probably should not be merged as is - FOR DISCUSSION!

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#### References to other Issues or PRs
Fixes #23749 
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#### Brief description of what is fixed or changed
Adds support for Mul instances as arguments to vector.cross. 

#### Other comments
This probably should not be merged as is - FOR DISCUSSION!

#### Release Notes

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[oscarbenjamin]

Rather than special casing Mul in cross it would be better to fix it so that expand still gives a VectorMul. The problem is the Mul._eval_expand_mul method which is inherited by VectorMul from Mul. That method calls fraction:

sympy/sympy/core/mul.py

Line 922 in edf2425

n, d = fraction(expr)

The fraction function turns the VectorMul into a Mul for the numerator. Probably VectorMul should override the _eval_expand_mul method to something that separates the scalar part of the VectorMul and recursively calls expand on that.