add dev container and tests

[john0isaac]

This PR adds DevContainer support following our render-engine/render_engine configuration in addition to setting up testing for the site generation with Cookiecutter and site build with render-engine-cli.

fix #18
fix #10

[john0isaac]

@kjaymiller I think render-engine is not compatible with python 3.9 😄

  File "/tmp/pytest-of-runner/pytest-0/test_site_generation0/.venv/lib/python3.9/site-packages/render_engine/page.py", line 29, in BasePage
    template: str | type[jinja2.Template] | None
TypeError: unsupported operand type(s) for |: 'type' and 'types.GenericAlias'

[kjaymiller]

one minor comment but this looks good. Free to merge!

[kjaymiller]

any reason not 3.13?

[john0isaac]

we did discuss this before it was not available at the time of our discussion.
let me check if it's available now

[john0isaac]

I think it's available now
https://github.com/devcontainers/images/tree/main/src/python

let me test it tonight.

[john0isaac]

they are lying the only available 3.13 is dev-3.13.
let's keep 3.12 until they release a stable 3.13

See:
https://mcr.microsoft.com/v2/vscode/devcontainers/python/tags/list