Query monitor: WordPress Playground compatibility (#215)

Follows up in #212. Accounts for the WordPress Playground runtime, where
the sapi name is `cli` and the `$wpdb` variable [may not be
available](https://github.com/WordPress/wordpress-playground/blob/c27a05386ce480c19a67be316d9bc093ffeec256/packages/playground/wordpress/src/index.ts#L391-L462).

## Testing instructions

Just confirm it doesn't break CI. We don't have an easy way of running
this PR in Playground (simply installing the plugin will trigger
different code paths).

Author: adamziel

integrations/query-monitor/boot.php

@@ -39,7 +39,9 @@
 }
 
 if ( 'cli' === php_sapi_name() && ! defined( 'QM_TESTS' ) ) {
-	return;
+	if ( ! defined( 'QM_RUN_IN_CLI' ) || ! QM_RUN_IN_CLI ) {
+		return;
+	}
 }
 
 if ( defined( 'DOING_CRON' ) && DOING_CRON ) {
@@ -56,6 +58,10 @@
 	}
 }
 
+if ( ! isset( $wpdb ) ) {
+	return;
+}
+
 // 2. Check if Query Monitor is active.
 if ( null === $wpdb->options ) {
 	global $table_prefix;