Now that the editor has their own public version of this API, and all
code has been migrated to use the new system, we can delete all of the
code that was supporting the old handlers. It may be surprising that
much of this code was dead, but the command handler list that was being
invoked by all of this was indeed empty.

The intent of this particular commit is to be a fairly mechanical change
that leaves other bugs or strangeness in place. In particular,
DebuggerIntelliSenseFilter had a special ExecuteVisualStudio2000 that
removes the buffer's read-only flag, but in many codepaths we never
set that back. In other cases, we also ignored the HRESULT that came
from the next filter in the chain. These all appear to be bugs, but I
won't touch them for now.

There's also some remaining work happening in
AbstractOleCommandTarget.Execute.cs that probably should either be moved
to the core editor code (if it wasn't already), or into the command
handlers of Roslyn itself.

Now that TypeScript has moved to the newer editor commanding APIs,
the legacy implementation is no longer needed.

0. Separate chaining and handling
1. Bound check
2. Add tests for bound and tab

Plus variable renaming

1. Drop "is" for some names
2. CommentBlock -> BlockComment

Plus a little clean up.