Add TypeScript API declaration file

[szkiba]

An index.d.ts file should be created in the root of the repository, which describes the extensions's JavaScript API using the TypeScript declaration file format.

Background

The k6lint and k6-extension-registry types checker checks if a TypeScript API declaration file (index.d.ts) for the k6 JavaScript extension exists in the root of the repository.

If index.d.ts does not exist, the extension will receive a lower grade. In this case:

See also:

• https://github.com/grafana/k6lint#checkers
• https://github.com/grafana/k6-extension-registry/wiki/Compliance
• https://github.com/grafana/k6-extension-registry/wiki/OKRs