Add a filter() helper method

Add a namespaced `filter()` method that can be imported via `use
function` and use it as an alternative for `new Filter` or
`Filter::create()`.

This provides a cleaner, easier to read syntax - particularly if you
have a lot of filters in the same file.

tests/Unit/HelpersTest.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+
+use Opdavies\GmailFilterBuilder\Model\Filter as FilterClass;
+use function Opdavies\GmailFilterBuilder\filter;
+
+final class HelpersTest extends TestCase
+{
+    /** @test */
+    public function it_returns_a_filter_object(): void
+    {
+        $this->assertEquals(new FilterClass(), filter());
+    }
+}