This replaces the use of strings for identifying messages by
types (WorksheetExecRequest and WorksheetPublishOutputNotification).

Also contains some light cleanup/uniformization

- Only enable worksheet features on the client if the server supports
them using the LSP support for dynamic feature registration.
- Replace the global functions and state in worksheet.ts by a
WorksheetProvider class, move some of the global functions specific to
one worksheet into Worksheet

We used interchangeably the term "run", "evaluate" and "exec" for
talking about running a worksheet. This commit does a lot of renaming
to standardize on using "run" everywhere.