selection handler.

* Implement pattern-matching in the nullable walker

Fixes #29909
Fixes #31881
Fixes #30952
Fixes #33499
Fixes #30597
Fixes #32414
Fixes #23944

* Remove infinite recursion by using an empty struct cache.

* Changes per code review comments.

* Remove debugging code accidentally left behind.

* Analysis of patterns-matching in the nullable walker requires valid (>0) slots.

* Skip a flaky test

* Patch after merge.

* Make ctor private to force use of factory methods

* Correct a typo.

* Fixup after merge.

* move StreamJsonRpc to 2.0

* use object and collection initializer

* changed to throw rather than return

* share JsonRpc creation when it can share code

* add linked files to share code

* more moving around so that we can have linked file working

* addressed PR feedback

* changed name to WatsonReporter

* moved to latest streamjson version

* added a way for partner to add thier own json converter to roslyn service hub service base type

* fixed test break.

it is caused by behavior changes between StreamJsonRpc 1.x and 2.x

* added comment follow PR feedbacks

* updated comment following feedbacks

Fixes #34472

* fixed issue where we crash due to our pending async work run after VS shutdown

this is another case where we have a pending async task that run after VS shutdown and it throws and our fail fast code catch that exception and crash VS.

general fix will be some thing like us making our fail fast code to aware shutdown situation and ignore any exception if we are in the shutdown situation.

but until we come up with proper design, this should handle one of high watson hits.

* PR feedbacks

When we completed a batch, we'd always raise a generic ProjectChanged
event, even if the change could be represented more precisely: if we
only added a single Document we could still raise DocumentAdded but
we'd still raise the generic ProjectChanged. This now raises more
precise events, when possible. This should allow for downstream
optimizations: adding a new document won't have to rerun syntax
analyzers on everything else, for example.

There was also a second issue where if the batch was closed and
nothing happened at all, we'd still raise a ProjectChanged. This was
because the old code tried having a special case where if it mutated
the Solution and ended up with the same Solution at the end, it'd
skip the actual change. However there was a bug where if it called
Solution.AddProjectReferences or Solution.AddMetadataReferences and
passed an empty list, it'd make a new solution snapshot. That's not
necessary to do, so we optimize that at the Workspaces layer.

Fixes https://github.com/dotnet/roslyn/issues/34309

A recent refactoring caused a number of insertion failures as we weren't
properly updating the contents of VS.ExternalAPIs.Roslyn.nupkg. This
adds basic verification that the contents are correct based on our build
output.

* Added CSharp.Scripting to VS.ExternalApis pacakge

* Add ExternalAccess to VS.ExternalApis package

The project which deploys ILASM tools ends up bringing two runtime
packages that have the same assets:

- runtime.win-x64.microsoft.netcore.runtime.coreclr
- runtime.win-x64.microsoft.netcore.app

Specifically assets like SOS.NetCore.dll exists at different versions in
these packages and end up getting copied twice to the output directory.
This double write ends up breaking our build (as well as basic
isolation).

The short term fix here is to no longer treat this as a code project but
instead a tools project. That eliminates the MS.NetCore.app package and
the associated double writes

Long term though the root issue needs to be addressed: making ilasm and
ildasm easier to deploy.

https://github.com/dotnet/coreclr/issues/15059

If you have an STA thread that created Runtime Callable Wrappers, it's
possible to end up in a deadlock where the finalizer and app domain
unload code are waiting for each other. The suggested workaround from
the CLR team is to wait for finalizers before letting the domain
shutdown but while the thread is still running.

Fixes https://github.com/dotnet/roslyn/issues/34248