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

/cc @dougbu 
:+1: to @pranavkm.  I'd go a bit further and add a comma after `SpanBuilders` and split the second sentence after `symbol`.  Perhaps "Thus" instead of "So" but that's a nano-nit.