Improve the way ehrql code is made available

[rebkwok]

Currently the postCreate script fetches ehrQL code from main. This means the ehrQL code can quickly go out of date with the ehrql image.
An alternative is to extract ehrql code from the current ehrql image, and add to the vscode PYTHONPATH. See POC repo. We can do this on the postAttach command so that the ehrql code is updated whenever a codespace is restarted.

In future, we might consider making this part of the vscode extension, which would let us check that the ehrql code is up to date e.g. when the active text editor changes to an ehrql file.