Built-in chr()

Add chr() built-in to convert numbers to unicode characters. Invalid values produce an empty string.

- single value: ```echo(chr(9786));``` -> ECHO: "☺"
- multiple values: ```echo(chr(97, 98, 99));``` -> ECHO: "abc"
- range: ```echo(chr([49:2:60]));``` -> ECHO: "13579"
- vector: ```echo(chr([46, 122, 90]));``` -> ECHO: ".zZ"
- invalid: ```echo(chr(-3));``` -> ECHO: ""