Add Deno + pg-mem doc

[oguimbal]

Added some doc, as suggested in #553

Is that what you had in mind ?

I also added a link to pg-mem a lib I recently released that uses Nearley to parse SQL (and execute it in-memory), if that's okay :)

[kach]

Thanks for doing this. Could you instead put just a sentence with a link under Tooling >> Other tooling? That's where all "third-party" stuff goes.

Adding the link to pg-mem is okay.

[oguimbal]

@kach No problem, done !

[kach]

Thanks!