Clean up pixi.toml

[emilk]

Our pixi.toml is a bit of a mess. We need to:

• Define what a lint is
• Define how we prefix commands
• Define how we order commands
• Replace fast_lint.py with pixi?

Needs

A few high-level commands capturing most of the things a regular users wants to do, including:

• building and running the viewer
• building and activating the python wheel
• running formatting and lints

after that comes the need of the CI which includes:

• Run checks for only one language
• more fine-grained control of the checks