Issues with `parseIdentifier` and escaped control characters

[JakeQZ]

During removal of support for IE hacks (#995), it was found that

• parseIdentifier() treats the first character differently to subsequent characters - it does not attempt to escape the first character;
• it escapes characters like TAB or NUL with the literal character, so the result is, e.g., a \ followed by an actual TAB, rather than \t or \9.

Perhaps these characters should not be allowed in identifiers anyway. Would need to check the spec, but I think the idea of the IE hacks was for IE-specific rules, so I expect these characters should be disallowed (as well as other control characters).

This was the test CSS:

p {
    padding-right: .75rem \9;
    background-image: none \9;
    color:red\9\0;
    background-color:red \9 \0;
    background-color:red \9 \0 !important;
    content: "red \9\0";
    content: "red\0abc";
}