Add schema version to kproj files
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.
Added TagHelperContext.UniqueId:
- The ID is created at view compilation time and is unique per TagHelperExecutionContext and thus per HTML element in the source for which Tag Helpers will run
- #241
Addressing CR feedback & adding unit tests
Added some more unit tests
- #241
Addressing CR feedback
- #241
Skip failing test
- #241
Addressing CR feedback:
- #241
Fixed test
- #241
Remove skipped test
- #241
Fixing tests
- #241