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.

src/Helpers.php

@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Opdavies\GmailFilterBuilder;
+
+use Opdavies\GmailFilterBuilder\Model\Filter;
+
+function filter(): Filter
+{
+    return new Filter();
+}