feat: support type conversion syntax

[YoEight]

Summary

Implements field AS TYPE syntax for explicit type conversion in EventQL queries. Supports built-in types (String, Int, Float64, Boolean, Date, Time, DateTime) and custom user-defined types.

Changes

Language Features

• New AS operator with highest precedence
• Case-insensitive type name matching
• Parser validates RHS must be a type identifier

Type System

• New type variants:
  • Type::Date - date values (e.g., 2026-01-03)
  • Type::Time - time values (e.g., 13:45:39)
  • Type::DateTime - datetime values (e.g., 2026-01-01T13:45:39Z)
  • Type::Custom(String) - user-defined types

API Changes

• AnalysisOptions::add_custom_type() - builder method for custom types

Error Handling

• ParserError::ExpectedType - when AS operator RHS is not a type
• AnalysisError::UnsupportedCustomType - when custom type not registered

Usage

// Built-in types
let query = parse_query(
    "FROM e IN events PROJECT INTO { date: e.data.date as DATETIME }"
).unwrap();

// Custom types
let options = AnalysisOptions::default()
    .add_custom_type("CustomTimestamp");

let query = parse_query(
    "FROM e IN events PROJECT INTO { ts: e.data.ts as CustomTimestamp }"
).unwrap();
query.run_static_analysis(&options).unwrap();