Improve deserialize return types

[Corb3nik]

HTTPQL return types for the deserialize function should be structured similarly to

ClauseRequest = { field: "FileExtension": expr: ...} | { "field":  ... }

This allows us to have exhaustive switch/case statements:

type X = { field: "A"; value: "B" } | { field: "C"; value: "D" };

const x: X = {};

switch (x.field) {
  case "A":
    x.value; // should be "B"
    break;
  case "C":
    x.value; // should be "D"
    break;
}