Improved User Experience with "Navigate to decompiled sources"

Right now we only recognize /shared and /keepalive for
these flags, but we allow '-' everywhere else as an option
prefix, so we should do the same here.

Code fix for CS8313, CS8363 (invalid default literal as case constant / pattern) to use another literal

Add support for default namespace to workspace

* 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

Merge dev16.0.x to master

Merge 'features/NullableReferenceTypes' into master

Remove "Make readonly" suggestion from fixed size buffers

Command handler to replace `@$"` with `$@"` when typing interpolated verbatim string