chore(gatsby): Convert db/nodes to TS

[pvdz]

Converts src/db/nodes to TS.

Note: first commit renames the file, second commit contains the changes. Github will consider it an added/removed file as a whole (because git does).

Also refactors src/utils/source-nodes since it now realizes the real typing of the getNode function and that it may return undefined. Had to change the loop logic a little to account for that.

Did not change the weird export mechanic that's going on here because that requires bigger refactors in other parts of the code. But this is basically a re-export that other consumers should import directly;

module.exports = {
  ...nodesDb,
  runQuery: runFastFiltersAndSort,
}

[pvdz]