Export `AsBox` method on `Envelope`

[peterstace]

Description

This is a helper method that converts a geom.Envelope to an rtree.Box. Without this helper, users have to call MinMaxXYs() to get the two extreme points from the Envelope, and then construct the rtree.Box manually.

Check List

Have you:

• Added unit tests? Yes.

• Add cmprefimpl tests? (if appropriate?) N/A

• Updated release notes? (if appropriate?) Yes.

Related Issue

• https://github.com/peterstace/simplefeatures/issues/513

Benchmark Results

N/A

[peterstace]

Thanks for reviewing!