Support UNION queries

[adamziel]

There's a proposal to add a UNION query to WordPress core: https://core.trac.wordpress.org/ticket/61097. Also, a fair share of plugins use UNION queries: https://wpdirectory.net/search/01HWQ4DT6Y1QP8W0DJ8GMZJJZ6

The SQLite integration plugin should support those, too.

To get there, we might need to resolve this issue first:

• https://github.com/WordPress/sqlite-database-integration/issues/106