Add fetch types shim?

[dsherret]

Nowadays, undici isn't necessary to use for Node, but still some global types error in Node when using fetch. There should probably be a built-in undici-types shim (this is what @types/node uses):

{
      package: {
        name: "undici-types",
      },
      globalNames: [{
        name: "BodyInit",
        typeOnly: true,
      }, {
        name: "RequestCache",
        typeOnly: true,
      }, {
        name: "RequestMode",
        typeOnly: true,
      }, {
        name: "RequestRedirect",
        typeOnly: true,
      }, {
        name: "ReferrerPolicy",
        typeOnly: true,
      }],
    }

Deps:

dependencies: {
      "undici-types": "^5.26",
    },