Thanks for the code pointer. However each debug adapter extension provides these variables, so I can not just add a list of these in the configuration editing extension. The right solution for me feels like each debug adapter extension would provide these completion proposal.

VSCode could also dynamically provide this list of proposols after collecting them from the adapter extensions however in vscode I can not get a reference to your json-parser, I only see it being used from extensions (thus I can not read the location).

Assigning also @weinand

@isidorn The vscode.d.ts API lets you get all extensions (extensions.getExtension), so you can actually find out about all contributed variables.
If you wanted to do this in the main process you can use vs/base/common/json. But I don't recommend that as we should not do too much parsing the main thread. Doing it in the 'configuration-editing' extension seems like the right place to do.
Doing it in every debug adapter extension is also possible, but it would mean that every extension would parse the file instead of just doing it at one place.

We now suggest variables inside values that are string for both the tasks.json and launch.json

@dbaeumer let me know if you do not like this behavior for tasks.json and I can disable it, though I find it very nice.

@isidorn like it as well. Thanks for working on it.