Add Headers::qfactors() to implement content negotiation

[thekid]

Example:

$preference= Headers::qfactors()->parse('deflate, gzip;q=1.0, *;q=0.5');
// ['deflate' => 1.0, 'gzip' => 1.0, '*' => 0.5]

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Content_negotiation

[thekid]

MDN writes:

The Accept-Encoding header defines the acceptable content encoding (supported compressions). The value is a q-factor list (e.g., br, gzip;q=0.8) that indicates the priority of the encoding values.

Should the method be named qfactors() or qualityfactors() instead?

[thekid]

Released in https://github.com/xp-forge/web/releases/tag/v4.4.0