feat: support optional parameters

[YoEight]

Summary

This PR adds support for optional function parameters in the EventQL type system. Functions can now define parameters as optional, allowing them to be called with a variable number of arguments within a specified range.

Changes

New Public API

FunArgs struct

A new public type that represents function argument types with optional parameter support:

pub struct FunArgs {
    pub values: Vec<Type>,  // All argument types
    pub needed: usize,       // Number of required arguments
}

Example

// Function with optional parameter: (boolean, number?) -> string
let args = FunArgs {
    values: vec![Type::Bool, Type::Number],
    needed: 1, // Only first parameter is required
};

assert!(args.match_arg_count(1));  // Valid: only required arg
assert!(args.match_arg_count(2));  // Valid: required + optional
assert!(!args.match_arg_count(3)); // Invalid: too many args

Built-in functions update

The COUNT aggregate function now supports an optional boolean parameter:

PROJECT INTO { total: COUNT() }

PROJECT INTO { voters: COUNT(e.data.age > 30) }

Of course COUNT parameter needs to be source-bound soCOUNT(false) will get rejected at static analysis time.