Add support for custom package manager.

[mysteryven]

I really like this theme, It's a great work! But I have a little suggestion.

I initialize my repo by npm init -y, but when I finished npx create-blogster@latest --theme sleek, it shows help information below:

Start your development with:

    - cd my-blogster-blog
    - yarn dev

Do you think add detection of package is a worth doing for? Maybe support of pnpm can speed up initialization.

[flexdinesh]

That'd be nice. PR welcome.