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.
Remove ContentBehavior.

- Removed all tests and instances of ContentBehavior in preparation for moving to a non-ContentBehavior based design.
- Removed ContentBehavior specific CodeGeneration.

#221
Re-design TagHelperOutput and runtime dependencies to allow all content modes.

- Added PreContent, PostContent and ContentSet properties to TagHelperOutput.
- Added GeneratePreContent, GeneratePostContent and SupressOutput methods to TagHelperOutput.
- Added multile ExecuteChildContentAsync and GetChildContentAsync to the rendering phase, ultimately only exposing GetChildContentAsync to a TagHelper author.
- Added more knowledge of StartWritingScope and EndWritingScope to the TagHelper runtime components. This is to enable the runtime components to utilize the RazorPage's infrastructure to render a delegate to a writer and retrieve its value to ultimately expose it to the user.

#221
Modify tests to allow new content mode design for runtime components.

- Added a new internal ctor for TagHelperExecutionContext since it's used in multiple tests to allow for less friction testing.

#221
Added CodeGen for TagHelper content mode redesign.

- Modified the CSharpTagHelperCodeRenderer to understand a single line of TagHelper rendering (instead of doing different things based on ContentBehavior).
- Modified existing CodeGen output to reflect new content changes.

#221
Rebased and addressed code review comments.