Introduce WP_Term_Query and use in get_terms().

`WP_Term_Query` is modeled on existing query classes, such as those used
for comments and users. It provides a more consistent structure for generating
term queries, and should make it easier to add new functionality in the future.

Props flixos90, boonebgorges.
See #35381.

git-svn-id: https://develop.svn.wordpress.org/trunk@37572 602fd350-edb4-49c9-b593-d223f7449a82

Author: boonebgorges