Implement "nitro run" command

[lo5]

Goal
Execute apps locally given one of:

• nitro run foo.py
• nitro run https://gist.github.com/user/hash

Notes
The deps for the app are self-contained in the .py file using a shebang-meets-dockerfile thingy:

# --- env ---
# USE venv
# RUN pip install pandas bokeh flask h2o-nitro
# RUN FLASK_APP=app.py flask run
# --- !env ---
def main():
    pass

or

# --- env ---
# USE venv
# FILE requirements.txt EOF
# pandas==4.5
# bokeh>=0.4.2
# flask
# h2o-nitro>0.8.2
# EOF
# RUN pip install -r requirements.txt
# RUN FLASK_APP=app.py flask run
# --- !env ---
def main():
    pass

Supported commands are

• USE uses venv, conda, poetry, pipenv, etc.
• FILE creates a file relative to cwd.
• RUN executes a shell command.
• GET fetches a file given a relative or absolute URL.
• COPY copies, moves, renames local files.

[lo5]

Language spec: https://github.com/h2oai/nitro/blob/main/docs/nitrogen.md

CLI docs: https://github.com/h2oai/nitro/blob/main/docs/cli.md