More cleanup to do between RoslynCompletionSet and
FilteredRoslynCompletionSet.

invoke next handler in case if no signature help providers can be found (i.e. if document for the text snapshot cannot be found)

Completion sometimes runs in buffer that haven't been registered for undo
with the editor, causing ITextUndoHistoryRegistry.GetHistory to throw an
exception. Call TryGetHistory to avoid the exception.

Also add support for completion item filtering.

This is the implementation assembly with implementation details that
we should not depend upon from our shipping binaries.

.. when completion can not have a session.

Avoid commiting the selected item when a commit character was typed at the start of a non-empty filter span.

Now that I have changed the flow for formatting here, the formatter
could return no changes. In those cases, don't update the document.

..chars from completion. We used to call the formatting service from the
completion list with the span equal to that of the text that we
inserted, in all cases. This may not be the right approach in all cases
and that is manifested when we are formatting for a typed char that
happens to be the trigger character for the formatter as well. In such
cases we must find the right span to give to the formatter, or in other
words, the span the formatter would have come up with if it were
triggered first for the typed char. We do this by simply invoking the
appropriate formatting service API and giving it the typed char and its
position, which lets the formatting service to compute the appropriate
span.
We do this only when the completion's commit char is also
a trigger for formatting. In other cases, we continue taking the
existing code path of formatting by giving it a known span.

Fixes #7768, #8228

Now, Add-Using will use the indices if they have been built.  Or not search
those projects/metadata if they're not available.

Note: this doesn't change any of the searching of the current project.  That
works the same way it always has.  This is only for searching unreleated projects
and metadata dlls.

Cleaning up some usings/Imports at the same time

This change makes foreground scheduling code predictable in the IDE
tests.  Instead of using a static to hold the data we use instance
values.  The static data the instance values are populated from are not
100% predictable but far more so than they used to be.

Add an interface, ICustomCompletionItemFormatter, that when implemented by a CompletionListProvider, is called to format the CompletionItem.

Years ago, we added a line of code to explicitly focus the editor when our ICompletionSession is dismissed, to handle a specific problem with VB completion. Essentially, through a subtle confluence of focus issues and events firing, VB line commit would run when double-clicking an item in the completion list to commit it. However, this line is causing numerous other problems in Visual Studio. (There's a particularly bad one when clicking VS toolbar item when the completion list is up.) After removing this line, I can no longer reproduce the original problem. So, I'm removing this to address the other problems. If it turns out that the original problem still occurs in certain circumstances, we'll work with the VS editor team to add a proper fix.

We were doing so to disable snippets in the interactive window, but it
turns out that it's per-user, rather than per-workspace.

Fixes #5393