Allow defined tables without exact rows

[Alex Vallone (amv146)]

Summary

• allow DefinedTable to retain a known column schema when exact rows are unavailable
• make the rows property optional while preserving rows: [] as an exact empty table
• allow callers to omit the definedTable factory's third argument
• continue validating the shape and types of every supplied exact row
• distinguish unknown rows from exact empty rows during type equality checks
• bump @microsoft/powerquery-parser to version 2.0.0 for the breaking type contract change

Testing

• npm run build
• targeted type utility tests
• targeted ESLint
• full library test suite (690 passing, 1 pending)
• full resource test suite (102 passing)
• npm pack --dry-run --json