Bug when self closing tag is before top level script/style

[dummdidumm]

Describe the bug
The current extractTag util has a bug:
parse5 is very html spec compliant, also when it comes to self-closing tags. That means that something like <SelfClosing /> <p>bla</p> will not parse as we would expect - the node would also span <p>bla</p> and more, until parse5 would see a real closing tag (</SelfClosing>).

The result of this bug is that it can mess up source mappings and therefore errors/warnings are shown in the wrong place.

Workaround

Don't use self closing components.

To Reproduce
Short Test that fails with the current implementation:

        it('can extract with self-closing component before it', () => {
            const extracted = extractTag('<SelfClosing /><style></style>', 'style');
            assert.equal(!!extracted, true);
        });

Expected behavior
Should work, because Svelte can also deal with it.

System (please complete the following information):

• OS: -
• IDE: VSCode
• Plugin/Package: Svelte Beta

Additional Information
We can maybe switch to using the svelte compiler's parse output once the parser is more error forgiving (sveltejs/svelte#4818)