Add pre-set-object-term-action

[geekofshire]

This PR introduces a new pre_set_object_terms action that triggers before terms are assigned to an object in wp_set_object_terms. Currently, it is only possible to access term taxonomy IDs ($old_tt_ids) after terms have already been updated, which limits the ability to make adjustments prior to committing changes.

Trac ticket: https://core.trac.wordpress.org/ticket/60532

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[apermo]

The filter itself looks good, but it needs to be moved up, this position looks broken to me as described.

[apermo]

I think this needs to be moved up by 15 lines above if ( ! $append ) {.

In line 2816, $object_id is already used, by altering $object_id in line 2842 this will normally produce $old_tt_ids that are no longer related to $object_id

[geekofshire]

Fixed in d382ac0

[apermo]

LGTM