Adds API to flow analysis to determine what variables were definitely assigned entering the region and which were definitely assigned when leaving. 

* Report rude edits when updating method containing switch expression.

Fix stackalloc rude edits.

* Updating modifiers of local functions

* await foreach, await using

* Improve error message

* Distinguish async foreach/using from synchronous in error messages

* Using declaration

* Report rude edit when editing using statement/declaration around non-leaf AS

* Rude edits for switch with patterns

* Fix VB issues with active statements in nested single-line functions and when updating iterators

* Update comment

* Fix duplicate rude edit

If you mouse over a local, field, etc., and we have computed nullable
flow analysis for it, we'll tell you whether the compiler thinks the
item is null or not at that point.

* Add Sort Usings command back to the Edit > IntelliSense menu

* Remove old style command args and registration

* Fix assertion message when missing a code action

* Added CodeRefactoringHelpers from Cyrus

* Added 'Introduce using statement' refactoring

* xlf

* Tests for 'Introduce using statement' refactoring

* Offer refactoring on vertical selection of declaration line

* Fleshed out tests

* Enable refactoring in switch sections and for invalid embedded declarations

* Removed accidental test complexity

* Enabled introducing using statement for VB

* Stop offering 'introduce using' on VB single-line If

* Added single-line lambda tests

* Document when it is that block helpers will be used

* Remaining review fixes

Added AddMissingImport CodeRefactoring driven by PasteTrackingService

Added PasteTrackingService that handles the paste command and track
paste information for documents. This paste information can be used
by refactorings to make suggestions based on what was pasted.

Fixes #10272

Add features to offer using conditional expressions ```?:``` over explicit if-statement flows. (#26236)

* Initial UseConditional work.

* Initial UseConditional work.

* Working replacement.

* Extract common code.

* Make 'var' work properly in the simplifier service.

* Move types to their own files.

* Provide a way to create types that are not simplified to 'var'.

* Update test options.

* Update test.

* Move CSharp code style code down to the Workspace layer.

* Rename config value.

* Add ui options.

* Add VB ui options.

* Add VB support

* Strip trivia

* Initial stubs for 'use conditional for returns'

* Add VB impl.

* Only support processing of real trees.

* Actually assign variable.

* Don't assume a single variable.

* Share more code.

* Fix shared code.

* Add check.

* Simplify code.

* Only the analyzer needs to check the severity.

* Generalize loc strings.

* remove static.

* Remove unneeded methods.

* Do cheap checks first.

* Handle more cases.

* Handle more cases.

* Add test.

* Add tests.

* Add test.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Fix tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Add tests.

* Format multiline conditions.

* Add multiline formatting for VB.

* Move code.

* Simpler formilization of casts.

* Extract out code.

* Remove file.

* Move files.

* Share more code.

* Share more code.

* Break out types.

* Share more code.

* Add more tests.

* Add VB tests.

* Simplify.

* Add comment.

* Remove newlines.

* Move method.

* Move method.

* Add comments.

* Simplify

* Update comment.

* Update comment.

* Remove line.

* Update comment.

* Update comment.

* Update comment.

* Fix comment.

* Wrap long lines.

* Wrap long lines.

* Wrap long lines.

* Handle trivia.

* Simplify.

* Fix check.

* better support for else-if chains.

* Add comments.

* Remove 'Use Conditional Expressoin'

* Remove more vestiges of
 feature.

* Remove code.

* Add back newline.

* foreach to for for C#

* use different method

* no ctor needed

* initial vb support

* added more tests in vb, more code share. more trivia tweak on vb

* more referrence cases

* handle embeded statement case

* some clean up

* addressing some feedbacks. some clean up and etc

* added more unit tests, more refactorings

* assigning better collection name

* more refactoring

* added comment

* more feedback updates

* removed old resource

* use semanticFacts.GenerateUniequeName

* changed to use TypeStypleHelper

* added style test

* fixed test failure

* fix up resx file.

* improve local variable name handling

* updated title.

removed from part.

* share duped code

* missed some places to convert to new dedup method

* more feedbacks

* fix up test failures due ot GenerateUniqueLocalName changes.

* Rename files.