Evaluate sharing code between xpath and xform-engine packages

## Description

In some cases, there is similar code between xpath and xform-engine packages, and it has been observed in codec functions, for example, geopoint ([xpath](https://github.com/getodk/web-forms/tree/main/packages/xpath/src/lib/geo) and [engine](https://github.com/getodk/web-forms/pull/300/files#diff-0abeb27e98642f30ee2753100cd5fccb8bd5bbfd86008290bd8780010c784f5e)) and dates ([xpath](https://github.com/getodk/web-forms/tree/main/packages/xpath/src/lib/datetime)) related logic.

Evaluate sharing code between xpath and xform-engine packages.

It could be in the common package or even a new package for codecs.

Compare the codec-related code in packages/xpath/src/lib/geo and packages/xpath/src/lib/datetime with the corresponding xform-engine logic referenced by pull request 300. First identify the duplicated geopoint and date behavior, then evaluate whether it belongs in the common package or a new codecs package. Done means a documented sharing approach and an agreed package boundary.

Written by the indexing model from the issue text.