Implement Mime sniffing to determine the resource mime type

[krichprollsch]

mime type

In browser.zig, the HTTP response's content type is base only on content-type HTTP header.
If the header is missing, we assume the content type is text/html.

But we should instead implement Mime sniff to detect the current document's mime type.

[krichprollsch]

relates with #509

[krichprollsch]

fixed by #553