Switching package manager

Switching package manager in an existing Nutin app requires adapting:

• breaking: packageManager in .nutin-meta.json (used by nutin-update and nutin-add)
• breaking: package.json scripts (serve, serve:prod, testin-nutin, testin-nutin:watch, testin-nutin:coverage, testin-nutin:verbose)
• breaking: tools/dev/dev-serve.js
• breaking: tools/dev/watcher.js
• breaking: tools/testin-nutin/watch-tests.js
• cosmetic: tools/generator/generator.js, tools/builder/builder.js, tools/utils/build-lock.js and nutin.config.js (messages and comments)
• breaking (if Docker feature is already added): tools/docker/Dockerfile.template