Parse and build a query string

Filters, search boxes, and pagination all live in the URL. Write the two functions that move between a query string and a plain object. Do not reach for URLSearchParams: the point is to handle the details yourself.

Implement parseQuery(text) and stringifyQuery(obj).

parseQuery returns an object of strings:

• A leading ? is allowed and ignored. An empty string gives {}.
• Keys and values are percent-decoded. A + in a key or value means a space, but %2B means a real plus sign.
• A key that appears more than once becomes an array, in the order it appeared. A key that appears once stays a string.
• a= and a bare a both give the empty string "".
• Stray & characters are ignored.

stringifyQuery is the reverse:

• Keys and values are percent-encoded with encodeURIComponent.
• An array value becomes one key=value pair per item, in order.
• Pairs are joined with &. There is no leading ?.

parseQuery("?tag=js&tag=css&q=hello+world&page=")
// { tag: ["js", "css"], q: "hello world", page: "" }

stringifyQuery({ tag: ["js", "css"], q: "a&b" })
// "tag=js&tag=css&q=a%26b"

Write two top-level functions named parseQuery and stringifyQuery. Do not use export or module.exports.