Merge 'dev16.0.x' into 'features/NullableReferenceTypes'

Change Windows build machine affinity to Windows.10.Amd64.ClientRS3.DevEx.Open (#29709)

* Change Windows build machine affinity to Windows.10.Amd64.ClientRS3.DevEx.Open

These machines have .NET Framework 4.7.2

'features/NullableReferenceTypes'

Merge dev15.9.x to dev16.0.x

Revert "Fix #30091 throwing exceptions"

This reverts commit b3ac2a6f.

Merge dev15.9.x to dev16.0.x

The general pattern when registering code fixes seems to be
either passing diagnostics[0] or diagnostics.First().

RemoveUnusedVariable seems to be the only outlier in this case,
doing .Single().

Having multiple diagnostics on a span would cause the code fix
to throw an exception

Add support to save current tools options into an .editorconfig file

default to vsn if NGenApplication is not set

Add test for Span not allowed as lhs of ??=

This ports https://github.com/dotnet/roslyn/pull/28472, that went into the `editorconfig-ide` feature branch, into `dev16.0.x` branch with following additional refactorings/enhancements:

1. Generated .editorconfig file has all localized string as comments.
2. No hardcoding of individual options in the editorconfig file generator. Instead, each `EditorConfigStorageLocation` now requires to provide a delegate to convert a given option value into the .editorconfig string representation (basically complimentary of the parsing delegate that was already required).
3. Introduce concept of `OptionGroup` to enable grouping options in the same feature (such as CodeStyleOptions, CSharpCodeStyleOptions, FormattingOptions, CSharpFormattingOptions, etc.) into sub-features or groups (such as "this. preferences", "expression-level preferences", etc.). This enables the editorconfig generation to group the related options, without explicit knowledge of each option group.
4. Expose `AllOptions` from each option feature (such as CodeStyleOptions, CSharpCodeStyleOptions, FormattingOptions, CSharpFormattingOptions, etc.), so that the corresponding `IOptionProvider` always stays in sync with implemented options.

Fixed IntelliSense suggestion for read-only class properties in object initializers 

Merge dev15.9.x to dev16.0.x

`InternalError` and `InsertMethodWithExplicitInterfaceSpecifier` were
both added with ID "80", but in different release trains.

`InternalError` was created as "80" for 15.9, so therefore we really
shouldn't change that as it would _definitely_ break SQM/telemetry/etc.

`InsertMethodWithExplicitInterfaceSpecifier` was created as "80" for
16.0, which hasn't officially shipped yet, so I'm changing that one to
"81". Now, that does mean any incoming SQM/telemetry/etc. will conflate
these for any interal builds of 16.0, but that's the least problematic
solution I could come up with.