Add new class for auto set vars to certain types or null

Convert\VarSetType for always return type expected (eg string will be string)
or Convert\VarSetTypeNull to return string or null on failure

The return value for failure if not matching type can be set for both, but
only for Convert\VarSetTypeNull the return value can be set to null.

4dev/tests/CoreLibsDebugSupportTest.php

@@ -114,7 +114,15 @@ final class CoreLibsDebugSupportTest extends TestCase
 	 */
 	public function printToStringProvider(): array
 	{
+		// 0: unput
+		// 1: html flag (only for strings and arry)
+		// 2: expected
 		return [
+			'null' => [
+				null,
+				null,
+				'NULL',
+			],
 			'string' => [
 				'a string',
 				null,
@@ -333,16 +341,19 @@ final class CoreLibsDebugSupportTest extends TestCase
 	 * @dataProvider printToStringProvider
 	 * @testdox printToString $input with $flag will be $expected [$_dataName]
 	 *
-	 * @param mixed $input
-	 * @param boolean|null $flag
-	 * @param string $expected
+	 * @param mixed $input anything
+	 * @param boolean|null $flag html flag, only for string and array
+	 * @param string $expected always string
 	 * @return void
 	 */
-	public function testPrintToString($input, ?bool $flag, string $expected): void
+	public function testPrintToString(mixed $input, ?bool $flag, string $expected): void
 	{
 		if ($flag === null) {
 			// if expected starts with / and ends with / then this is a regex compare
-			if (substr($expected, 0, 1) == '/' && substr($expected, -1, 1) == '/') {
+			if (
+				substr($expected, 0, 1) == '/' &&
+				substr($expected, -1, 1) == '/'
+			) {
 				$this->assertMatchesRegularExpression(
 					$expected,
 					\CoreLibs\Debug\Support::printToString($input)