TypeScript checking

[lukehesluke]

Rationale

Changes to Data Model Validator are propagated to 1). The user-facing validator.openactive.io 2). Other libraries and tools like Test Suite.

For this reason, it's important to be sure not to break things. TypeScript, which we already use in other projects, and can be easily integrated into JS-only projects via JSDoc (which thus also improves documentation)

Suggestion

• Add TS checking as part of npm test
• Set coverage to include as much as possible, but exclude any files that have too many errors. Try to include at least fundamental files
• Update the contributing guide to suggest adding new rules to tsconfig.json include list