WIP: Drop node 12, add node 18

[mroderick]

The failure from FormData is caused by jsdom-global, which removes it in the cleanup.

See https://github.com/rstacruz/jsdom-global/blob/master/keys.js#L122

The jsdom-global looks pretty dead. A quick fix would be to fork it and improve it to not destroy the new globals in Node 18.... or maybe there are better libraries out there now.

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The FileReader failures are because the FileReader API is not available in node, and newly added global Blob support is enabling the codepaths that use FileReader.

https://github.com/nodejs/node/pull/41270

We should probably learn how to read blobs in a node native way, not using FileReader, and save that for browsers.