Update Razor to use official xunit runner
Add line mappings to project TagHelper attribute values.

- We now create LineMappings for instances where a TagHelper's attribute value is not of a string type.
- This will enable the Razor editor to create projections from .cshtml => .cs for TagHelper attributes.
- Modified the TagHelperCodeGenerator and TagHelperBlockBuiler to accurately track the attribute values start locations so it could flow into code generation.
- Modified existing tests to account for the new line mappings.

#207
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