Address #236, recreate XML files when xml-docs-test target is run directly
- must still `build initialize` between `git clean` and `build xml-docs-test`

nit: add description of xml-docs-test target
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.

#215
Change TagHelperDescriptor to 1 ctor so it can be deserialized.

Made existing ctor that was only ever used in Razor tests to be internal.
Updating to release NuGet.config
Updating to dev NuGet.config
Expose GetExportedTypes on TagHelperTypeResolver.

- This will enable tooling to provide their own mechanism for resolving assemblies.
Expose TagHelperDescriptors on TagHelperBlock.

- This will enable tooling to determine which descriptors are relevant to which TagHelperBlocks in the syntax tree.
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
Addressed code review comments.