Merge master to dev15.6

Fix getting package references service

Override object.ToString in Optional<T>

We're trying to go from an `IVsBrowseObjectContext` to an `IPackageReferencesService`.
We were doing this through `IVsBrowseObjectContext.ConfiguredProject`, but it turns out
that this property always returns null on the particular implementation of
`IVsBrowseObjectContext` we're working with.

Instead, we need to go through the `UnconfiguredProject` and ask it for the "suggested"
`ConfiguredProject`.

I've also switched from `GetAwaiter().GetResult()` to properly async/await code inside
an `ExecuteSynchronously` call, mostly for the purposes of readability.

Move remainder of Roslyn to new SDK

Set insert_final_newline to true for C# and VB source files

Since tests are run in a subfolder of the solution, they can currently see
the changes we make to .editorconfig.

Introduce a more flexible service to allow languages to plug into common comment/uncomment behavior

Add entries for dev15.3.x branch in readme.md

Don't assume a document has SyntaxFactsService

Skip CSharpInteractive.TypingHelpDirectiveWorks test

Improve performance of inline rename with multi-targeting

If it doesn't, just use case sensitive matching

Use Capacity instead of Count to identify large ArrayBuilder instances

Don't fetch every PR ref for every PR job

The runtimeconfig.json files are only necessary on CoreClr applications. The SDK by default generates it for all project types. This leads to unnecessary output in desktop projects that gets included in items like VSIX. Disable the generation to prevent this from happening.

The paths in MSBuild use a mix of `/` and `\\`. The normalization to one
or the other happens very late in tool task execution. It's still very
visible during execution of most tasks in our targets files.

This means the comparison in BuildUtil must account for this.

The NuGetPackageToIncludeInVsix element is used to make the NuGet assets
we use in a VSIX very explicit. It depended on the nuget items having
the metadata element NuGetPackageId. The 1.X New SDK no longer has this
piece of metadata and hence we had to pick a new approach.

After some discussion decided to simply look for the assets which are
coming out of the NuGet packages folders that we expect. It's
essentially the same check.

Needed to encode this in an MSBuild task though as the MSBuild
transforms can't handle that level of complexity.

https://github.com/dotnet/project-system/issues/2488

By default the new SDK will cause projecs to reference the full
transitive closure of project references, not just the direct
references. This is fine in most cases but caused breaks in our VSIX
projects.

Our VSIX projects often reference project with specific extra metadata
to control whether or not their contents are included in the final VSIX.
That extra metadata is not applied to projects which are implicitly
referenced by the new SDK. That caused our VSIX to bloat in size and
have functionally incorrect contents.

Fixed this by disabling implicit transitive references for our VSIX
projects.

https://github.com/dotnet/sdk/issues/1366

The warning B40057 used to be suppressed at a global level in PCL /
Desktop projects. The SDK does not suppress this warning and hence
converting to the new SDK caused this to show up.

Previously a number of developers fixed this problem individually. The
correct solution is to suppress it globally as was done previously.