Handle unclosed and invalid structure HTML tags for TagHelpers.

- Added detection of unclosed tags (tags without begin/end).
- Added recovery of potentially unclosed tags.
- Added detection of invalid structure tags (tags that do not end with '>').
- Modified detection of bad attribute values to be parse errors instead of runtime errors.
- Modified RazorParser to sort errors. This made writing tests more intuitive and ultimately ensures that the editor shows errors in the correct order.
- Added tests to validate invalid tag structure.
- Added tests to validate invalid attributes.
- Added tests to validate unclosed tags.

#104