Use JSON destination for fetching source maps

[nicolo-ribaudo]

I was working on using fetch to define how source maps are loaded, for #30, and I did some analysis on the current HTTP headers to understand which parameters to pass to fetch.

Firefox devtools:

Headers {
  accept: "*/*",
  "accept-encoding": "gzip, deflate, br",
  "accept-language": "en-US,en;q=0.7,it;q=0.3",
  "cache-control": "max-age=0",
  connection: "keep-alive",
  host: "localhost:4507",
  "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:121.0) Gecko/20100101 Firefox/121.0"
}

Chrome devtools:

Headers {
  "accept-encoding": "gzip, deflate, br",
  "accept-language": "en-GB,en-US;q=0.9,en;q=0.8",
  connection: "keep-alive",
  host: "localhost:4507",
  "sec-fetch-dest": "empty",
  "sec-fetch-mode": "no-cors",
  "sec-fetch-site": "same-origin",
  "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0."... 17 more characters
}

Safari devtools:

Headers {
  accept: "*/*",
  "accept-encoding": "gzip, deflate",
  "accept-language": "en-GB,en;q=0.9",
  connection: "keep-alive",
  host: "localhost:4507",
  referer: "http://localhost:4507/",
  "sec-fetch-dest": "empty",
  "sec-fetch-mode": "no-cors",
  "sec-fetch-site": "same-origin",
  "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17."... 19 more characters
}

I noticed that both Firefox and Safari use Accept: */*, even if the source map is expected to be a JSON file. We recently introduced a "json" fetch destination (https://fetch.spec.whatwg.org/#dom-requestdestination-json) that would have the following effects:

• Accept is application/json,*/*;q=0.5
• For browsers that support it, Sec-Fetch-Dest is json

It is currently used by the JSON modules spec import ... with { type: "json" }, but it would fit great for this use case too.