ux: list tasks when command is not found (#488)

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