`scribble/html/lang' to `scribble/html'.
Since `foo:' keywords are very common in any code that uses
`scribble/html', it is much more convenient to have it provide the
custom `#%top'. It simplifies code that uses it, for example, files in
`meta/web/common' now use
#lang at-exp racket/base
(require scribble/html)
instead of the (only-in scribble/html/lang #%top) which it used before,
or instead of explicitly quoting all `foo:'s.
