This reverts merge request !166

This reverts commit b3fb1a16

It introduced changes which cause issues described in
gargantext/purescript-gargantext#571

This commit adds a bunch of Arxiv tests to demonstrate we can
successfully transform our `Query` type into the Arxiv AST.

It's not fully wired in, `convertQuery` for Arxiv is implemented as
`undefined`.

This commit simplifies the `ExternalAPIs` type, which is now a pure
enumeration, moving the lookup of the Pubmed key at the usage sites.
This simplified the code quite a bit, and made some otherwise-effectful
functions pure.

Also, added a basic GraphQL endpoint.

Based on the suggestions in
https://kodimensional.dev/space-leak