Reuse functions from `export://explorers/covid/latest/covid` in other explorer steps

[Marigold]

COVID explorer config file and helper functions look very helpful. We should put them in explorer_helpers.py to make them available to others.

We should also add JSON schema and update .vscode/settings.json to match files from export/explorers/**/*.yml (chatgpt is very good at creating schemas from given yaml files).

cc @lucasrodes