Implement column metadata (#209)

This PR implements emulation of `PDOStatement::getColumnMeta()` values
and adds support for `$wpdb::get_col_info()`.

This feature is necessary for support of MySQL tools like Adminer,
phpMyAdmin, and others, as well as for any plugins that may rely on this
functionality.

Resolves
#190.

Author: JanJakes

.github/workflows/wp-tests-phpunit-run.js

@@ -13,7 +13,6 @@ const path = require( 'path' );
 const expectedErrors = [
 	'Tests_DB_Charset::test_invalid_characters_in_query',
 	'Tests_DB_Charset::test_set_charset_changes_the_connection_collation',
-	'Tests_DB::test_get_col_info',
 ];
 
 const expectedFailures = [