Improve `lerp` robustness

[peterstace]

Description

This change makes some tweaks to the lerp function that make it more robust under certain edge cases.

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/447

[peterstace]

Thanks for reviewing!