Use TypeScript

[borgar]

• Port the project to TS
• Use vitest as the test runner (Switch to vitest #78)
• Use typedoc to generate documentation
• Clean up interfaces
  • Methods now return undefined where they previously returned `null``
  • Token type values are now uppercase:'zero' became 'ZERO'.
  • tokenTypes was removed, replaced by 34 importable TOKEN_* constants.
  • The CONDITION token value changed from ['>=', '10'] to '>=10'.
  • Locales are now dash delimited rather than underscore: "fi-FI" instead of "fi_FI".