Export combine_elements()

[dmuenz]

This PR aims to fix #6882.

ggplot2 does not have an exported function that resolves a theme element object against a parent element object, applying the same resolution logic as calc_element(). In particular, such a function should correctly resolve deferred values like rel() and margin_part(). However, an unexported function named combine_elements() does exactly this -- it is the function that calc_element() uses under the hood.

So this PR simply (1) exports combine_elements, (2) improves its documentation since it will now be user-facing, and (3) for the same reason changes the function's parameter names from e1 and e2 to child and parent. As a bonus, (4) I added some new unit tests to check that deferred value inheritance works.

Note that an exported function named merge_element() exists and appears very similar to combine_elements(). But merge_element() does not resolve deferred values between a child and parent element object. I address this in the revised documentation for combine_elements().

BTW, I'm open to other name ideas for what I'm calling "deferred values", i.e., the return values of the rel() and margin_part() helpers that don't fully specify a value. I use this term in the documentation.