Function parse_expr 

pub fn parse_expr(input: &str) -> Result<DiceExpr, ParseError>

Parse a dice expression string into a DiceExpr.

Supports expressions like "2d10+4", "adv d20+5", "dis d20-1", "2d6+1d4+3", and "4d6kh3" (keep highest 3 of 4d6).

Errors

Returns a ParseError if the expression is malformed.