fix: correct optional peer dependencies

[alumni]

Currently optional peer dependencies are not declared correctly, they are declared twice: as mandatory peer dependencies as well as optional direct dependencies.

This often causes false positives during security scans. E.g.: preview-email is not needed in production, however it has often been the cause of failing security audits.

This PR aims to declare them according to the package.json spec, i.e. using peerDependenciesMeta. This is available since npm@6.11 (August 2019), so any package manager in use nowadays should be able to handle this:

• npm: https://docs.npmjs.com/cli/v10/configuring-npm/package-json#peerdependenciesmeta
• yarn classic: https://classic.yarnpkg.com/en/docs/package-json#toc-peerdependenciesmeta
• yarn berry: https://yarnpkg.com/configuration/manifest#peerDependenciesMeta
• pnpm: https://pnpm.io/package_json#peerdependenciesmeta

With this change, apps relying on @nestjs-modules/mailer will be able to avoid installing some of these optional peer dependencies if they are not needed (e.g. via pnpm.ignoredOptionalDependencies).

[alumni]

@juandav Could you please review? Thank you :)