Add found TagHelperDescriptors on a Razor page to GeneratorResults.

- Ultimately this enables tooling to inspect what TagHelperDescriptors were found on a document and construct HTML schema based off of them.
- Added XML doc on the classes I touched that didn't have docs.
- Added [NotNull] to the result construct parameters.
- Added tests to validate that TagHelperDescriptors flow when found after parsing a Razor document.

/cc @dougbu @pranavkm 

#215
not great to have a supposedly-optional concept such as tag helpers front-and-centre in `GeneratorResults` and the base `ParserResults`.  made worse by breaking compatibility and adding the required `IEnumerable<TagHelperDescriptor>` parameter to all existing constructors.

would prefer to make this information available somewhere else entirely.  if that's not possible, add a `{ get; set; }` property to one of `ParserResults` or `GeneratorResults` and revert changes to the other class.