LibWeb: Add DOMParser

This allows you to invoke the HTML document parser and retrieve a
document as though it was loaded as a web page, minus any scripting
ability.

This does not currently support XML parsing.

This is used by YouTube (or more accurately, Web Components Polyfills)
to polyfill templates.

Userland/Libraries/LibWeb/HTML/DOMParser.idl

@@ -0,0 +1,6 @@
+interface DOMParser {
+    constructor();
+
+    // FIXME: "type" should use the DOMParserSupportedType enum.
+    Document parseFromString(DOMString string, DOMString type);
+};