Separate `Envelope` validation from construction

[peterstace]

Description

This change moves Envelope validation out of constructors, and into a separate Validate method. This is similar to the recent changes for regular geometries.

It includes two further small breaking changes, so that all breaking changes are bundled together:

• The ExtendToIncludeXY method is renamed to ExpandToIncludeXY (to be consistent with ExpandToIncludeEnvelope).

• The NewEnvelope now accepts a variadic list of XY rather than a slice of XY. This is just a small ergonomic improvement.

Check List

Have you:

• Added unit tests? Yes.

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

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

Related Issue

• N/A

[peterstace]

Thanks for reviewing @albertteoh 🙇