feat: add a pixi file and split into two envs (host & build)

[wolfv]

I was trying to do some development on lpython.

It might simplify the first steps for some people :) The lockfile should create a consistent environment. I only have tested this on macOS arm64 so far though.

We create two environments:

• compiler: contains cmake, ninja, and the C++ compiler
• host: contains llvmdev and zlib (and whatever else might be linked)

A start task is also added to run the development version of lpython. I was pretty confused for a while which runtime folder to include so I also added that to the start command (and changed build1.sh).

I can fix this up for linux and windows if you all like it.