feature/implement lexer - part 1

[yarkhinephyo]

What is in the PR

Implement part 1 of lexer. From this paper, the items of table I in page 5 is added to tests/lexer/README.md. A checklist is added to track what has been implemented and what is still remaining.

tldr;

Tokens in the table implemented in the PR

• BOOL - Boolean literal TRUE | FALSE
• NUMBER - Integer, floating point, or scientific notation [0-9]+ .? [0-9]* (e [0-9]+)?
• STRING - String literal " ([^ "] | "")* "
• ERROR - Error literals #NULL! | #DIV/0! | #VALUE! | #NAME? | #NUM! | #N/A
• ERROR-REF - Reference error literal #REF!

Tokens not in the table but used in BNF form implemented in the PR

• Array Delimiters
• Unary Operators
• Binary Operators

How is it tested

For each entry, added unit tests in tests/lexer/test_**.rs.

cargo test

[qwoprocks]

Took a quick look, I realized that we are missing one thing, which is that all formulas should start with =

[qwoprocks]

niceeee