Corrected literal string highlighting.

Literal strings (single-quote strings) were having some sequences
incorrectly treated as escape sequences.

The only valid escape sequence in a literal string is a double
single-quote sequence, which expands to a single, single-quote
character.

For example the literal string:

    'To insert a newline character, use the ''\n'' escape sequence.'

Is equivalent to the regular string:

    "To insert a newline character, use the '\\n' escape sequence."

This commit fixes this behaviour.