ux: list tasks when command is not found

[ruben-arts]

Closes #485

Helps user with:

➜ pixi run bla
bla: command not found

Available tasks:
        build
        build-docs
        docs
        install
        lint
        test
        test-all

➜ 

Allows user to:

➜ pixi run 'echo $PIXI_PACKAGE_VERSION'
0.7.1-dev

➜ pixi run 'echo $PIXI_PACKAGE_VERSION && echo $PIXI_PACKAGE_NAME && echo $CONDA_PREFIX'
0.7.1-dev
pixi
/home/rarts/development/pixi/.pixi/env

Depended on denoland/deno_task_shell#98

[baszalmstra]

What happens if you pass 'echo "HELLO WORLD!"'? 🤔

[baszalmstra]

Maybe you should shlex::split? https://docs.rs/shlex/latest/shlex/fn.split.html

[ruben-arts]

This is already parsed as two things and the pixi run "echo 'Hello World'" also works but that becomes weird.

But to miss edge cases we didn't think of now that shlex lib looks like a beter way forward!