Default values for generic parameters

[ssalbdivad]

Would allow parsing the equivalent syntax from TS:

const array = type("<t = unknown>", "t[]");

// unknown[]
const a = array();
// string[]
const b = array("string");

const types = scope({
  "array<t = unknown>": "t[]",
  // unknown[]
  a: "array",
  // string[]
  b: "array<string>",
}).export();

[ssalbdivad]

Should also add defaulted generic parameters for some of our existing instance types like Set, Map and Array.