Replace yargs-parser with Node.js built-in util.parseArgs

[mcollina]

Removes the yargs-parser dependency in favor of Node.js native util.parseArgs, which is available in Node.js 18.3+ and 20.0+.

Changes

• New lib/parse-args.js module wraps util.parseArgs with camel/kebab normalization, boolean flag consumption, short aliases, env prefix resolution (FASTIFY_\* environment variables), number coercion, and --` separator handling
• All CLI argument parsing now uses this module instead of yargs-parser
• Removed yargs-parser from package.json dependencies

Breaking Changes

• --help / -h is now a boolean flag instead of consuming the next argument as its value. Previously -h true would set help to the string 'true'; now -h simply sets help to true and true becomes a positional argument.
• Plugin options passed after -- separator: unknown --key value pairs now parse --key as a boolean (true) instead of taking value as its string value

Testing

All existing tests pass with updated expectations.