Support adding multiple packages with add command

[r-bt]

While following the tutorial at: https://docs.tscircuit.com/tutorials/build-a-custom-keyboard-with-tscircuit, it says to run:

tsci add @tsci/seveibar.PICO @tsci/seveibar.Key

However this throws an error:

error: too many arguments for 'add'. Expected 1 argument but got 2.

Using the variadic argument <packageSpecs...> in the commander definition resolves this.

Note that adding multiple packages through the interactive cli (i.e. running tsci and then selecting add from the menu) did work.

However, this highlighted a bug with the addPackage function where it was only normalizing the first package in a list of packages. So I added a new function addPackages which normalizes all the packages, and have addPackage just call that.

All the existing add testcases pass, and I added a new one to verify that adding two packages works

[rushabhcodes]

also update in readme

[r-bt]

also update in readme

Done