fix(loader): update parse boolean to work with all accepted yaml values

[joshfried-aws]

Enhanced YAML Boolean Parsing for CloudFormation Guard

Summary

This PR enhances the YAML loader to properly handle all standard YAML boolean values. The implementation now correctly recognizes the full range of YAML 1.1 boolean representations
including:

• true, yes, on, y (and their case variations) as boolean true
• false, no, off, n (and their case variations) as boolean false

Problem

Previously, the loader was using Rust's standard parse::() which only recognizes "true" and "false" strings. This limited functionality prevented CloudFormation Guard from correctly
parsing YAML files that use alternative boolean representations that are valid in the YAML specification.

Solution

The implementation now includes:
• A custom boolean parser that handles all YAML 1.1 boolean representations

Testing

Added extensive unit tests that verify:
• All valid YAML boolean representations are correctly parsed
• Case insensitivity is properly handled
• The parser correctly distinguishes between boolean values and other types

[mrinaudo-aws]

LGTM

[racicot-amzn]

LGTM!