Hello, first of all: Thanks for the amazing project, it allows me to use a lot of scientific python packages, that I would have to re-write in Dart otherwise :)

I hope that I did not overlook something, but it seems like there is no implementation for running the python app on the web platform yet. This PR solves that situation.

About my approach:

• Package the python app to a .zip as previously
• Unzip the package in the asset directory and add all relevant paths to the asset section in pubspec.yaml
• Running SeriousPython.run on the browser does the following:
  • It adds the pyodide.js library from https://cdn.jsdelivr.net/pyodide/v0.27.2/full/pyodide.js to the html file
  • Then, all required packages are read from the requirements.txt file and imported via pyodide
  • All additional modules are loaded (other python files, if provided)
  • I am using the environment variables to declare, which command should be executed (to enable a "server-like" architecture)
  • The result is read in Dart from a pyodide_result variable that needs to be assigned with the command result and returned as a string

I used this synchronous command approach, because for me, it does not seem to be possible to use Flask inside of a browser environment.

I also know that my approach can definitely be improved, but I unfortunately do not have the time currently to do so. I still wanted to create the PR to get feedback on the general approach and to enable community efforts :)

Possible improvements:

• Code cleanup (serious_python_web.dart got a bit messy with using Dart, java script and python all in one file...)
• Possibly: Do use the .zip file instead of the unpacked archive by using unpack_archive from python: https://pyodide.org/en/stable/usage/loading-custom-python-code.html
• Improve documentation and stability
• I am not sure if the existing Pyodide package command is actually required, because when improting the additional packages, packages that cannot be loaded are ignored anyway: https://github.com/biocentral/serious-python/blob/cc6d296dd06ad265e3bf97021368a9952987a89e/src/serious_python_web/lib/serious_python_web.dart#L78 But maybe I also got the idea wrong, how I should use the virtual environment from within pyodide...
• I am not able to test if my changes had any impact on mobile (Android, iOS) platforms at the moment

I am grateful for receiving any feedback!