Create Lexer w/ preliminary parsing logic

[keon-dev]

Perform lexical analysis to turn plain text source code into tokens

The lexer will receive input that looks like:

"let x = 5 + 5;"

And what comes out looks something like:

[
    LET,
    IDENTIFIER("x"),
    EQUAL_SIGN,
    INTEGER(5),
    PLUS_SIGN,
    INTEGER(5),
    SEMICOLON
]

Initial lexer should be able to parse the following snippet:

let five = 5;
let ten = 10;

let add = fn(x, y) {
    x + y;
};

let result = add(five, ten);

The following must be implemented to evaluate the above code:

• All whitespace must be skipped
• Numbers will just be integers and treated as an integer type
• Variable/function names will be "identifiers"
• The keywords should not be grouped together since it should make a difference in the parsing stage whether we encounter let or fn