fix: add type support for instance has plugin

Author: audothomas

test/types/instance.test-d.ts

@@ -326,6 +326,8 @@ const versionConstraintStrategy = {
 expectType<void>(server.addConstraintStrategy(versionConstraintStrategy))
 expectType<boolean>(server.hasConstraintStrategy(versionConstraintStrategy.name))
 
+expectType<boolean>(server.hasPlugin(''))
+
 expectAssignable<DefaultRoute<RawRequestDefaultExpression, RawReplyDefaultExpression>>(server.getDefaultRoute())
 
 expectType<FastifySchemaCompiler<any> | undefined>(server.validatorCompiler)

types/instance.d.ts

@@ -131,6 +131,7 @@ export interface FastifyInstance<
   hasDecorator(decorator: string | symbol): boolean;
   hasRequestDecorator(decorator: string | symbol): boolean;
   hasReplyDecorator(decorator: string | symbol): boolean;
+  hasPlugin(name: string): boolean;
 
   addConstraintStrategy(strategy: ConstraintStrategy<FindMyWayVersion<RawServer>, unknown>): void;
   hasConstraintStrategy(strategyName: string): boolean;