Changes

Summary

1. split veriT reconstruction into Lethe and veriT part
2. clarified options;
3. more robust errors -- on foreground process instead of background server;
4. clarified options -l vs. -R;
5. command-line arguments for "isabelle vscode", similar to "isabelle jedit";
6. proper command-line tool;
7. support console output, e.g. "isabelle vscode -C -- --help";
8. run Isabelle/VSCode via Scala;
9. clarified module name;
10. clean build from explicit MANIFEST: avoid accidental garbage in vsix package;
11. incorporate build_grammar into build_vscode_extension;
12. removed old generated file;