Adds a public API, `NullableContext GetNullableContext(int position)`, for determining the nullable context at a particular file location, and types supporting that API.

Implements https://github.com/dotnet/csharplang/issues/2389
Fixes #35958

Note that the switch expression conversion is not a standard implicit conversion.  Specifically, it should not be considered as a valid implicit conversion as input to a user-defined conversion, as that could permit two chained user-defined conversions.

Also changes binding of switch expression to ECMA so it can interact properly with switch expressions.
Fixes #14965

* Add optional [warnings | annotations] keyword to nullable directive

* pragma warning <setting> nullable -> nullable <setting> warnings

* Implement /nowarn:nullable

Add initial support for 'notnull' generic type constraint.

See https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-15.md
Also fixes #36005.

Support re-abstraction of interface members in derived interfaces

Related to #32054.

This was missed when we were adding another parameter to the one with
optional arguments.

Update the stub API to remove Nullability and replace with NullableAnnotation and NullableFlowState, and update the
the rewriter to use that instead of the Nullability enum.

* Add initial support for specifying explicit base type in the base access.

Relevant LDM notes https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-11-14.md .

* PR feedback

Adds a new "Preview" language version for C# that means the latest
supported language version + the preview features available in the
compiler. Also changes the "Default" (unspecified) language version to
mean Preview, but provides a warning when preview features are used.

Fixes #32290 
Fixes #32169 
Fixes #32141 
Fixes #32025 

* Rename "deconstruct pattern" to "positional pattern" in the APIs.
Fixes #32291

* Permit a trailing commas in two places
- Permit a trailing comma after the last arm of a switch expression
- Permit a trailing comma after the last subpattern of a property pattern clause
Fixes #32292

Merge remote-tracking branch 'origin/features/editorconfig-in-compiler' into merges/master-to-features/editorconfig-in-compiler

* Add syntax for await using var x = ...
- Update syntax nodes
- Add parsing tests

* Support binding and lowering for await using var x =...
- Add awaitable info to bound node
- Find and store the awaiter if async
- Lower the declaration node correctly
- Add tests

Don't warn about "!", regardless of context (https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-29.md).

Closes #30177.
Closes #30021.

Fixes #367

* Use a contextual UnderscoreToken for the discard pattern.
* Implement an error for `case _:`
Fixes #27750