Use of locale-sensitive methods with `undefined` locale may cause environment-sensitive bugs

Describe the bug

I've mentioned this before tangentially in a couple of issues (ex), but it's probably worth its own issue.

Use of locale-sensitive methods, such as toLocaleLowerCase, with an unspecified (undefined) locale may cause environment-sensitive bugs, because the default locale is host defined.

Steps to Reproduce

For example, in http/cookie.ts, which is marked browser-compatible:

https://github.com/denoland/std/blob/6a4eb6cb9185b0b9e0816a227c04b5bf039b99ef/http/cookie.ts#L361

Open Firefox

Go to https://codepen.io/lionel-rowe/pen/rNEXmoG?editors=0010:

import { getSetCookies } from 'https://esm.sh/jsr/@std/http@1.0.6/cookie'

const setCookie = 'a=b; EXPIRES=Thu, 19 Sep 2024 07:47:28 GMT'
const headers = new Headers({ 'set-cookie': setCookie })
const cookie = getSetCookies(headers)[0]

document.querySelector('#target').textContent = JSON.stringify(cookie, null, 4)

Observe expected output (assuming your locale isn't Turkish):

{
    "name": "a",
    "value": "b",
    "expires": "2024-09-19T07:47:28.000Z"
}

Go to about:preferences
Set language to Turkish ("Türkçe"; Turkish for "language" is "dil" if you need to find the setting again)
Go back to or refresh https://codepen.io/lionel-rowe/pen/rNEXmoG?editors=0010 page

Observe output:

{
    "name": "a",
    "value": "b",
    "unparsed": [
        "EXPIRES=Thu, 19 Sep 2024 07:47:28 GMT"
    ]
}

Expected behavior

Consistent behavior in all locales

Environment

OS: Windows 11
deno version: N/A
std version: @std/http@1.0.6

[x] http/cookie
[x] http/negotiation
[ ] text

denoland / std

Use of locale-sensitive methods with `undefined` locale may cause environment-sensitive bugs #6016