Make IgnoreOrder a func rather than a variable

[peterstace]

Description

The variable was a global variable, and could (theoretically) be changed by any user, altering its behaviour.

Check List

Have you:

• Added unit tests? No, relies on existing.

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

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

Related Issue

• https://github.com/peterstace/simplefeatures/issues/481

Benchmark Results

N/A.

[peterstace]

Thanks for reviewing!