Q# notebook magic command syntax highlighting support

[adrianleh]

Current situation:
When using Q#, one can create Jupyter notebooks that run python and then change cells to contain Q# code with a magic command %%qsharp in the first line of a cell (see https://docs.microsoft.com/en-us/azure/quantum/how-to-python-qdk-local#the-qsharp-magic-command). Right now, this causes there to be not proper syntax highlighting as VSCode still tries to highlight code as python. While there is an option to change the language mode (see screenshot below), Q# is not an option to change to.
If one opens up a notebook as a pure iqsharp, the syntax highlighting works as expected. Though, in this mode users cannot include python code interacting with Q# code. Such interoperation is often required for classical operations (i.e., preparation of quantum program, data visualization, etc.)

Desired behavior:
Ideally, VSCode would recognize the %%qsharp magic command and change the syntax highlighting of a given Jupyter cell to Q#. Barring that, it would be nice to have an option to switch cells manually to Q# highlighting.

[rebornix]

Moving to Jupyter for now as the magic command is a Jupyter knowledge. To support this we might need changes or APIs from VS Code core though.

[rchiodo]

See #4588

[rchiodo]

Adding the relevant data to the original issue. Closing as a dupe. Thanks.