By having the "is this a use strict directive?" logic in
parse_string_literal() we would apply it to *any* string literal, which
is incorrect and would lead to false positives - e.g.:
"use strict" + 1
`"use strict"`
"\123"; ({"use strict": ...})
Relevant part from the spec which is now implemented properly:
[...] and where each ExpressionStatement in the sequence consists
entirely of a StringLiteral token [...]
I also got rid of UseStrictDirectiveState which is not needed anymore.
Fixes#3903.
https://tc39.es/ecma262/#sec-directive-prologues-and-the-use-strict-directive
A Use Strict Directive is an ExpressionStatement in a Directive Prologue
whose StringLiteral is either of the exact code point sequences
"use strict" or 'use strict'. A Use Strict Directive may not contain an
EscapeSequence or LineContinuation.
