Implement JSON5 Mode

[nzakas]

Changes to be made:

JavaScript

• Object keys may be an ECMAScript 5.1 IdentifierName.
• Objects may have a single trailing comma.
• Arrays may have a single trailing comma.
• Strings may be single quoted.
• Strings may span multiple lines by escaping new line characters.
• Strings may include character escapes.
• Numbers may be hexadecimal.
• Numbers may have a leading or trailing decimal point.
• Numbers may be IEEE 754 positive infinity, negative infinity, and NaN.
• Numbers may begin with an explicit plus sign.
• Single and multi-line comments are allowed.
• Additional white space characters are allowed.

Rust

• Object keys may be an ECMAScript 5.1 IdentifierName.
• Objects may have a single trailing comma.
• Arrays may have a single trailing comma.
• Strings may be single quoted.
• Strings may span multiple lines by escaping new line characters.
• Strings may include character escapes.
• Numbers may be hexadecimal.
• Numbers may have a leading or trailing decimal point.
• Numbers may be IEEE 754 positive infinity, negative infinity, and NaN.
• Numbers may begin with an explicit plus sign.
• Single and multi-line comments are allowed.
• Additional white space characters are allowed.