Implemented tokenise_literal_string

One thing I've realised is that even methods such as this require
error tracking.  I won't implement it in the tokenise method as it's
not related to consuming the string per se but instead in the main method.

asm/lexer.cpp

@@ -296,3 +296,13 @@ pair<token_t, lerr_t> tokenise_literal_char(string_view &source, size_t &column)
   }
   return make_pair(t, lerr_t::OK);
 }
+
+token_t tokenise_literal_string(string_view &source, size_t &column, size_t end)
+{
+  source.remove_prefix(1);
+  token_t token{token_type_t::LITERAL_STRING, string(source.substr(1, end - 1)),
+                column};
+  source.remove_prefix(end);
+  column += end + 1;
+  return token;
+}