Changes

Summary

1. tuned;
2. publish extension on Visual Studio Marketplace;
3. updated VSCode extension to "isabelle vscode_server";
4. merged
5. DiagnosticSeverity according to implementation https://github.com/Microsoft/vscode-languageserver-node instead of documentation https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md;
6. support for syslog messages; clarified shutdown;
7. prefer stable state -- reduce repeated diagnostics;
8. more uniform pending_input / pending_output; explicit Document_Model.uri; tuned;
9. unused;
10. precise full_range and thus proper try_restrict in Snapshot.cumulate;
11. clarified protocol errors;
12. clarified signature;
13. unused;
14. clarified options;
15. clarified signature: maintan Text.Length within Line.Document;
16. clarified modules;
17. clarified signature: explicit Length to avoid implicit mistakes;
18. tuned;
19. more uniform treatment of input/output wrt. client; support for diagnistic messages; misc tuning;
20. tuned;
21. more uniform treatment of "bad" like other messages (with serial number);
22. clarified modules;
23. support for diagnostics;
24. print constants in 'primrec', 'primcorec(ursive)', 'corec(ursive)', like in 'definition' and 'fun(ction)'