Add Flux from() and filter() methods

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

src/Function/Base.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Function;
+
+abstract class Base {
+
+	/**
+	 * @throws FunctionNotImplementedException
+	 */
+	public function __toString()
+    {
+		throw new FunctionNotImplementedException('__toString', get_class($this));
+	}
+
+}

src/Function/Filter.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Function;
+
+class Filter extends Base {
+
+    /**
+     * @var array $settings
+     */
+    private $settings;
+
+    public function __construct(array $settings)
+    {
+        $this->settings = $settings;
+    }
+
+    public function __toString()
+    {
+        return '|> filter(fn: (r) => ' . implode(' ', $this->settings) . ') ';
+    }
+
+}

src/Function/From.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Function;
+
+class From extends Base {
+
+    /**
+     * @var array $settings
+     */
+    private $settings;
+
+    public function __construct(array $settings)
+    {
+        $this->settings = $settings;
+    }
+
+    public function __toString()
+    {
+        return 'from(' . implode(', ', $this->format($this->settings)) . ') ';
+    }
+
+    protected function format(array $settings)
+    {
+        array_walk($settings, function(&$value, $key) {
+			$value = $key . ': ' . (is_string($value) ? '"' . $value . '"' : $value);
+		});
+
+        return $settings;
+    }
+
+}

src/Function/FunctionNotImplementedException.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Function;
+
+use Exception;
+
+class FunctionNotImplementedException extends Exception {}