This Pull Request fixes/closes #778

It changes the following:

• Handle invalid Unicode code point in the string literals
• Fix the unexpected behavior of handling hexadecimal and Unicode escape sequence

The lexer will no longer panic when processing the string literal with invalid Unicode code points, e.g. "\uD83D". It will replace all the invalid code point with a fixed replacement char "�" ("\uFFFD"). For example, the string "ABC\uD83D123" will become "ABC�123" after processing. The replacement is not revertable so the following example will not have the expected behavior:

let x = '\uD834'; // '�', invalid code point
let y = '\uDD1E'; // '�', invalid code point
let z = x + y; // '��', the expected result should be '𝄞'

This issue can only be solved when we have the string data stored in bytes instead of chars.

This PR also fixes the unexpected behavior of handling hexadecimal and Unicode escape sequence, including:

'\xZZ'             // now throw a syntax error
'\uD834\u{{DD1E}}' // correctly make it a 4-byte character
'\u00AA\n'         // correctly handle Unicode escape sequence followed by other escape character