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 
super nit, split the comment into two lines (or more if you're creative). 

> When symbols are accepted into SpanBuilders their locations get altered to be offset by the parent. We account for this by marking our start location prior to adding the symbol so we know the location of the attribute value start within the document.