fix(types): improve Root shorthand method return types

[mahmoodhamdi]

The Root-level shorthand methods (Joi.allow(), Joi.valid(), Joi.required(), etc.) currently declare Schema as their return type. Since these methods delegate to this.any()[method]() at runtime (source), they always return an AnySchema. Using the broader Schema union type loses this specificity and breaks method chaining in TypeScript — e.g. Joi.allow('x').required() doesn't resolve to AnySchema.

Changes

• Updated return types for all Root shorthand methods (allow, valid, equal, invalid, disallow, not, required, exist, optional, forbidden, preferences, prefs, custom) from Schema to AnySchema
• Added missing only() and strip() declarations to the Root interface — these are registered at runtime but were absent from the type definitions
• Added type tests verifying method chaining works correctly with the new return types

Before

const s = Joi.allow('x');  // Schema (union type — no .required() etc.)
const s2 = Joi.required(); // Schema

After

const s = Joi.allow('x');            // AnySchema — supports chaining
const s2 = Joi.allow('x').required(); // AnySchema ✓
const s3 = Joi.only().valid('a');     // AnySchema ✓
const s4 = Joi.strip();              // AnySchema ✓

All tests pass: npm test (1796 tests, 100% coverage, types, lint).