transform: Add transform_vector methods (#24319)

# Objective

There isn't a particularly obvious way to transform a vector using a
`Transform` (you have to make an Affine3 first, and then use the matrix)

## Solution

This adds a `transform_vector` method to `Transform` and
`GlobalTransform`, paralleling the `transform_point` method.

## Testing

Added doctests.

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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Kevin Chen <chen.kevin.f@gmail.com>