Add ts declarations

This merge request ensures that the ucam-observe-remote-client package is fully compatible with TypeScript projects by including the compiled /dist folder. Previously, the absence of TypeScript declaration files caused import failures in strongly-typed projects.

Changes include:

1. Integrated npm run build into the install_dependencies job to ensure that the /dist folder, containing the .d.ts types, is consistently created and published to the npm registry.
2. Added dist/ to cache paths in .gitlab-ci.
3. Added .eslintignore.

These steps guarantee that the package is immediately usable with type safety in TypeScript environments upon installation.

A part of: https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/card-frontend/-/issues/230