Highlighting generated from the compiler's keyword lists; diagnostics from the compiler
run in place (NULL lextype); outline, definition, hover, signature help, references,
rename (including GLOBALS files); completion of variables, record fields, like-table
columns (from the compiler), SQL tables/columns/DDL, form fields and keywords valid at the
cursor (from the parser state); fgllint diagnostics; a debugger (gdb's DAP server behind a
4GL-aware proxy, using #line directives). Tests: node --test (unit and real compiler) and
npm run test:vscode (inside a real VS Code). Roadmap in TODO.md.