Add pyproject.toml, allowing for pipx install from github

[Jasha10]

This PR adds a pyproject.toml file to the project, which enables the project
to be installed via pip or pipx. This means users won't have to fiddle
around with cloning the repo and installing the dependencies manually.

In addition to adding pyproject.toml, this PR also:

• renames gpt.py to gptcli/__main__.py
• creates a new convenience executable shell script gpt at the top level of the project
• Adds an argument prog="gpt" to the ArgumentParser in __main__.py. This way running gpt --help will display the correct program name gpt instead of displaying __main__.

Testing

To test this out using pipx or pip, run the following commands to install from
the to-upstream branch on my fork of this repo.

# using pipx
$ pipx install git+https://github.com/jasha10/gpt-cli@to-upstream
$ gpt
Hi! I'm here to help. ...

[kharvd]

Very nice, thanks!

[kharvd]

Ended up incorporating the changes separately here #46