alpha() should reject non-filter-like arguments

[nex3]

The alpha() function allows some unquoted strings as arguments for backwards-compatibility with support for IE's old filter syntax, but in keeping with Ruby Sass and Dart Sass, only unquoted strings that begin with the regular expression [a-zA-Z]+\s*= should be allowed. All other strings should be rejected. Currently LibSass accepts any unknown values and passes them through unmodified.