Bump the version number to 2.3.1

Ensure we clean up RuleSetFiles when we close the solution

The VisualStudioRuleSetManager is a singleton that acts as a cache
of RuleSetFiles and also manages the lifetime of file watching. We
never cleaned that up when we shut down VS, so we leaked a bunch of
file watchers, which would throw during finalization asserting they
weren't cleaned up and bring down the process with it.

Change CoreXT package publishing

Use IVsSolution to get the correct IVsProject for an IVsHierarchy

Removing the old style of package publishing. The official build definition will now control
uploading packages to the DevDiv feeds.

Revert https://github.com/dotnet/roslyn/pull/20558

Revert "Merge pull request #20558 from Pilchie/Fix19968-DuplicateSourceAndAdditionalFiles-DistinctIds"

This reverts commit 6f5b2d47, reversing
changes made to 66362df5.

Fixes crashes associated with InvisibleEditor

Fix crash in element access signature help

Restrict Document acquisition to UI Thread

Don't force editor composition during ASL

If the user closes a textview while completion is filtering, it's possible for the filtering task to ask for a Document before it gets cancelled, resulting in a crash. To prevent this, restrict Controller.GetDocument() to the foreground thread.

Support having a file be both a document and additional file.

Don't crash in OOP with TS projects/documents

Change nuget moniker for dev15.3.x to beta4

This is a breaking API change.  Need to decide if we want to take it or
not.

Turns out we could end up with the same IVisualStudioHostDocument
for both, if they are both added.

Fixes #19968.

since this branch is now targeting Preview 4.

When lowering local functions that capture struct frames, the frames are
added as ref parameters to the lowered method. Normally, when the local
function captures variables from multiple levels up the intermediate
closures are also rewritten to take the ref parameters. However, if one
of the intermediate closures captures no variables, the closure
conversion code can skip adding ref environments to that closure. This
can cause the inner-most closure to no longer have access to all of the
ref structs it expects, which crashes the compiler.

Fixes #18814, #18918

Merging master into dev15.3.x for final stabilization

Add entries for dev15.3.x branch in readme.md

Don't assume a document has SyntaxFactsService