ebib--get-field-highlighted: replace `object-intervals'

[Hugo-Heagren]

Fix #275.

This is a slightly hacky way of providing compatibility with versions of Emacs without object-intervals. I've done this (rather than properly refactoring the code) because I think that in a few years, when `object-intervals is part of the emacs most people are using, it would be a good idea to go back (the operation is relatively slow and the current code is ugly, so going to a native solution which does exactly what we need, written in C, would be perfect).

[joostkremers]

@Hugo-Heagren Thanks, BTW, for providing such a quick fix!