Improvement: auto-sync extensions with devcontainer.json

[StefanoFrazzetto]

Hi everyone, I hope you're doing well!

First of all, thank you for your work; I'm enjoying testing Codespaces and I hope my feedback will be useful!

I just thought that it would be great if we could install and remove extensions, and have Codespaces automatically add or remove them from extensions, keeping the env in syc with devcontainer.json.

The current process for adding extensions

1. Install extension
2. Verify the extension works
   assuming the user knows the button exists
3. Go back to the extension's page in 'Marketplace'
4. Click the button 'Add to devcontainer.json'

Potential issues with that process

• I need to rebuild the container, but I forget to add the extensions to my devcontainer.json. After rebulding it, I notice all the extensions have vanished and I don't remember what I need or don't need;
• I'm trying a few new extensions and notice one of them doesn't work, or I just don't need it, but I already added it to my devcontainer.json. Now I'll have to find it in the list within the config file, remove it from there (there is not button to remove the extension, unlike step 4), and then also uninstall it. If I forgot to remove it from the config, it would be back on the next rebuild.

I understand one of the reasons for rebuilding a container is to restore the env to a previous known, working state, so maybe this could be something users can manually choose to enable. What do you think?