Add ref var decl detection for for/foreach

Unskip tests around tuple nullable annotations

Now that https://github.com/dotnet/roslyn/issues/36047 is fixed, unskip
type inferrer tests.

Ensure that IVsContainedLanguage.SetHost(null) won't fail a second time

Remove the AbstractIndenter type (and subtypes).

Add feature to introduce a local variable immediately after writing its initializer.

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

Move to latest FlowAnalysis Utilities assembly

* Fix EnC debug information emitted for patterns

Update calculation of syntax offset to account for a new case when a node (a switch expression) that is associated with a variable, closure or lambda may share start offset with other node of the same kind (`expr switch { … } switch { … }`). Use the offset of the `switch` keyword instead of the starting offset of the expression to disambiguate.

Assign ordinals to variables synthesized for storing pattern values across cases. This is required to support complex patterns since we can no longer rely on the type of these variables to be distinct. This will require follow up in the IDE to disallow updating/adding/reordering the case clauses of switch expression which there an active statement is present within the switch statement. If the cases are unmodified the compiler guarantees that the order in which the synthesized variables are generated remains the same, so we can map the variables using their ordinal.

Mark all variables synthesized during lowering of switch expression as short-lived. Their lifespan is limited to the switch expression, which does not include a sequence point.

Disallow editing methods that contain switch expression. This is necessary until bugs https://github.com/dotnet/roslyn/issues/37232, https://github.com/dotnet/roslyn/issues/37237 are fixed.

* Feedback

* Update tests

Use pattern matching in a few more places.

Fix significant wrong translation in CodeStyle.Resources.zh-Hans.xlf

Will be consumed by tooling to recommend LangVersion 

* Add all nullable-related warnings to ErrorFacts

* Fix test failures and add a bit more coverage

* Emit tests

* Report rude edits for unsupported DIM operations

* Allow EnC tests to specify TFMs to build against

* EnC: test against netcoreapp30

* Test fixes

* Fix merge

Fixes #36643
Fixes #37065

Expand introduced using statement to include encompassed locals

Fixes #36241

Fixes #37187