Index script

[woutslabbinck]

Split scripts/createIndex.ts into two:

1. creating the index
2. add it to jest (that has to succeed)

Furthermore add some documentation.
E.g. that it is used to create the index file about all the test cases locally such that other applications (such as FORCE) can use this index file together with the tooling provided in this repo to fetch test cases anywhere on the web.