Add window() function to Query Builder

Signed-off-by: davidarendsen <davidarendsen@hey.com>

tests/Functions/WindowFunctionTest.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Functions;
+
+use Arendsen\FluxQueryBuilder\Functions\Window;
+use PHPUnit\Framework\TestCase;
+
+final class WindowFunctionTest extends TestCase
+{
+    public function testSimpleWindow()
+    {
+        $expression = new Window('20s');
+
+        $query = '|> window(every: 20s) ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+    public function testAllParameters()
+    {
+        $expression = new Window('20s', 'every', '0s', 'location', '_time', '_start', '_stop', true);
+
+        $query = '|> window(every: 20s, period: every, offset: 0s, location: "location", ' .
+            'timeColumn: "_time", startColumn: "_start", stopColumn: "_stop", createEmpty: true) ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+}