Allow generating inline SVG code

[MatTheCat]

Type::getSvgCode returns a DTD (which is discouraged) and an XML declaration which are fine for a standalone files, but not for inlining.

Changing Type::getSvgCode signature to public function getSvgCode(bool $xmlDeclaration = true, bool $dtd = true): string would keep BC and allow to generate SVG code for all use-cases.