Add uninstall routine to cleanup the database

uninstall.php

@@ -0,0 +1,30 @@
+<?php
+/**
+ * Uninstall the plugin.
+ *
+ * Deletes the custom database tables, and the plugin options.
+ *
+ * @package ProgressPlanner
+ */
+
+// If uninstall not called from WordPress, then exit.
+if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
+	exit;
+}
+
+require_once __DIR__ . '/includes/class-settings.php';
+require_once __DIR__ . '/includes/class-query.php';
+
+// Delete the plugin options.
+delete_option( \ProgressPlanner\Settings::OPTION_NAME );
+
+// Delete the custom database tables.
+global $wpdb;
+// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
+$wpdb->query(
+	$wpdb->prepare(
+		// phpcs:ignore WordPress.DB.PreparedSQLPlaceholders.UnsupportedIdentifierPlaceholder, WordPress.DB.DirectDatabaseQuery.SchemaChange
+		'DROP TABLE IF EXISTS %i',
+		$wpdb->prefix . \ProgressPlanner\Query::TABLE_NAME
+	)
+);