Listen for workspace registration changes to ensure we add/remove the…

Fixes the bug reported very kindly by @martinstenhoff at #40066 (comment).

Fixes #41826, same bug

The debugger will use this to delay load Roslyn EnC until its needed.

Merge release/dev16.5 to master

Fix sorting for import completion

Fixes #41702

This reverts commit f19214b69c2fbc6aa8431e3aceeb8d805bb99c6f.

This reverts commit 9ef11ce3bcd1ddd60391060962158c2f79215170.

This is the version used in VS 16.5 preview 2 (d16.5 16.5.29721.120)

Merge release/dev16.5 to master

Make unit testing analyzer a singleton

We have two inaccurate terms left over from the closure
conversion rewrite: "lambda rewriter", which is now wrong
simply because it doesn't just rewrite lambdas, and "closure",
which was improperly conflated in the initial work to mean both
all nested functions, and the lowered form post-closure conversion
with all captures removed.

This change attempts to unify the naming for clarity. Local functions
and lambdas are now referred to as "nested functions" and "closure"
refers to the post-rewrite lowered method + environment(s).

Allow batched document removals and update some consuming code accordingly

CheckContainsProject asserts the project exists, no need to duplicate
it.

The implementation in SolutionState.RemoveDocumentsFromMultipleProjects
is heavily inspired (i.e. copy/pasted and tweaked appropriately) from
AddDocumentsToMultipleProjects.

This base type doesn't really make things "simpler" if the uses still
have static readonly lambdas. It may reduce things by a few lines of
code but that's not really simpler.

Remove an unused parameter

Address issues with nullable flow analysis slot container types and changing types during inference (#39301)

Fixes #39220
Fixes #39297
Fixes #33428

* Bind expression variables appearing in a goto case statement.
Fixes #40714