Implements most of the design changes specified in
https://github.com/dotnet/csharplang/blob/c229cae634bd59a6a13b9ed464a4cab782a95e5d/proposals/index-range-changes.md

This PR focuses on getting the simple end-to-end scenario working,
not focusing entirely on codegen quality. I expect to follow-up later
with the optimizations mentioned about eliminating use of the
Index/Range helpers entirely if they can be elided.

* Don't complete when multiple chars selected

* Undo change to pos return value

* Update config

* Disable some tests on Mac. Only have a mac coreclr job.

* Move the DeeplyNestedGeneric bar way down for Mac

* Add work item attributes for disabled tests

Fixes #34233

Don’t suggest upgrading to an unsupported state

Make sure to add the new namespace for declaring documents in IChangeNamespaceService

Improve naming of integration test runs

Specially handle type parameters constrained to `Nullable<T>` in TypeSymbolExtensions.CanContainNull(). (#34939)

Fixes #34679.

The shorter job name means the matrix name is visible in the list.

Revert "Format `{}`"

Merge dev16.1-preview2 to master

Fixes #34905

Fixes #34882

Fixes #33359

Type parameters with an oblivious constraint shouldn't be considered as PossiblyNullableReferenceTypeTypeParameter (#34899)

Fixes #34892

add razorSupport2 to published bits so that razor team can pick up bits from feature branch (#34884)

Revert "Regex Completion + Async Completion = Failure to trigger on `[` in VB"

Improve assertion message on quick info test failure

Restore packages after creating a CPS project in tests

Fixes #34165

This reverts commit 094dd50d.

Remove unnecessary code: was When generating constructor, trim known prefixes from argument name to get parameter name  (#34804)

* Fixes #33673

* Respond to feedback

* Remove unneeded code and add tests

* Fix access modifier

Remove verbose flag

Clean up our CI setup code

* Add SelectAsArray, DangerousCreateFromUnderlyingArray

* Add SerializableBytes.ToImmutableArray

Use CancellationTokenExtensions.CombineWith where appropriate