Add inv dev for local testing

[utensil]

inv dev will

Serve the blueprint website, rebuild PDF and the website on LaTeX file changes

To test the blueprint locally, if you have a working LaTeX environment, all you need to do is

sudo apt install graphviz libgraphviz-dev
pip uninstall -y leanblueprint
pip install -r blueprint/requirements.txt

to set things up.

Then run inv dev than navigate to http://localhost:8080/ to check the blueprint.

After each edits to the LaTeX, it will automatically rebuild the blueprint, you just need to refresh the web page to see the rendered result.

The caveat is, if you have something wrong in your LaTeX file, and the LaTeX compilation fails, the inv dev might stuck, and you'll need to use X then return to exit LaTeX, fix the LaTeX error, then run inv dev again.