feat: Change open to take a boolean | string

[atomflunder]

Hi again,

this is in response to the Issue I opened #199, I went ahead and implemented it, I hope it looks okay. I was not so sure about how to design some aspects, especially the getURL and generateURL helper functions.
If this is not something you wish to implement, feel free to close this PR and the underlying issue ☺️

As it stands, this will do:

• pnpm listhen . -> opens nothing
• pnpm listhen . --open -> opens the baseURL (e.g. https://localhost:3000/)
• pnpm listhen . --open myTest -> opens the baseURL + supplied string (e.g. https://localhost:3000/myTest)

resolves #199