Introduced in #945.

This is an edge case when a comment is closed twice leading to the
second end marker leaking to the next token.

This is an edge case when local `@import` was given by hash and
referenced a remote one which was processed synchronously but should
have been asynchronously.

When `animation-duration` is default but `animation-delay` isn't we
shouldn't drop the former as both need to be given.

When a value has `!important` in it then it should be stripped bare
before matching to the list of at-rules.

- case of roundBracketLevel inferior to 0.

`font` shorthand needs `font-size` and `font-family`.

Jamie Davis (@davisjam) from Virginia Tech reported that clean-css
suffers from ReDOS vulnerability [0] when fed with crafted input.

Since not so many people use clean-css allowing untrusted input such
cases may be rare, but this commit reworks vulnerable code to prevent
such attacks.

It also limits certain whitespace blocks to sane length of 31 characters
in validation regexes to prevent similar issues.

[0] https://snyk.io/blog/redos-and-catastrophic-backtracking