Validate no conflicting TagHelper attributes

[NTaylorMullen]

This involves ensuring that TagHelpers do not conflict with each other when it comes to HTML attribute names. EX:

class Foo : TagHelper
{
   int MyProp { get; set; }
}

[TagName("Foo")]
class Bar : TagHelper
{
    string MyProp { get; set; } // Conflict because it expects string and not int.
}

[dougbu]

note this is about catching errors prior to compilation of the generated code. could punt this one if we deem compilation errors sufficient for now.