feat: allow --watch to accept optional directory or list of directories

[josephreilly22]

Closes #787

Allow --watch to accept an optional directory or space-separated list of directories. When provided, only those paths are watched; when omitted, behavior is unchanged (watch process.cwd()).

Usage:

• fastify start app.js --watch or fastify start -w app.js → watch cwd (unchanged)
• fastify start --watch api api/server.js → watch only api/
• fastify start --watch "api lib" server.js → watch only api/ and lib/

Changes:

• args.js: Parse --watch as string, normalize and expose watchPaths
• start.js: Pass watchPaths to watch module
• lib/watch/index.js: When watchPaths is provided, watch only those dirs (resolved from cwd); otherwise watch cwd
• help/start.txt and README.md: Updated docs and example
• test/args.test.js: Updated expectations for watchPaths, added tests for --watch api and --watch "api lib"
• test/watch.test.js: Added test for watch with watchPaths

Backward compatibility: Existing --watch (no value) and config/env watch: true unchanged; watchPaths is [] and cwd is watched.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct