add support for Chain-of-Thought

It would be great if we could support COT like below

```
grammar_str = """
root ::= cot | phone_number

cot ::= [ a-zA-Z0-9]* phone_number

phone_number ::= "+" [0-9]+
"""
```

or for arithemetic

```
root ::= cot | result

cot ::= [^[">>>"] ]* result # We exclude ">>>" from chain of thought to make it a special marker

result ::= ">>>" [0-9]+
```

This way, we could have a prompt like

`Follow this example 3+3*4=3+12=15 >>>15; 97*(12-4)+9=`

and expect a generation like `97*8+9=776+9=785 >>> 785`