feat: add a form-urlencoded decoder with bracketed nesting

[hughgrigg]

Stacked on #23, which adds the decode hook this fills in.

application/x-www-form-urlencoded is a wire format, and not one service's dialect. Stripe, Rails
and PHP applications all speak it, all three write nesting the same way, and none of them could be
simulated without a decoder for it. It belongs beside JSON for the same reason JSON is here.

line_items[0][price_data][unit_amount]=250&line_items[0][quantity]=1&expand[]=customer

{
  "line_items": [{ "price_data": { "unit_amount": "250" }, "quantity": "1" }],
  "expand": ["customer"]
}

Decisions

Every leaf stays a string. A form body carries no types, and guessing at them would make
quantity=1 and postcode=01234 disagree about what a digit is. The resource's own creation
behaviour converts what it needs, where the target shape is known.

Digits order the entries and do not position them. a[0], a[5] and a[9] give three elements
and never a sparse array of ten. A real encoder counts from zero, where the two readings agree, and
this needs no cap on how large an index a hostile body may claim.

Four inputs throw a SyntaxError rather than being guessed at.

Body	Refused because
name=a&name=b	Which value wins is a guess
a=1&a[b]=2	a would hold a value and more keys at once
a[][b]=1	An empty bracket appends, and only the last part can
a[b, [a]	Brackets cannot be read out of the key

That follows the existing rule that decode failures stay loud. A real encoder emits well-formed
keys, so these are hostile or mistaken input, and a simulation quietly corrupted by them would be
worse than one that stops.

The content-type header is ignored. What a body claims to be and what it holds are two facts,
and choosing the decoder by hand has already settled the first.

Notes

A __proto__ key lands as an own data property and reaches no prototype, which Object.fromEntries
gives for free. There is a test pinning it.

The parser is a page of code and should stay one. Sparse arrays, dotted keys and qs compatibility
flags are each a decision a reader would have to hold in their head, and none is needed to simulate
an API.