Debt: Support launch.json creation through extension host code

[weinand]

Currently an extension can only contribute an initial launch.json statically, so it cannot adapt the launch.json to the contents or structure of an existing project folder.

For node.js we work around this in VS Code by specific code that understands the package.json.
With this feature we could move this code from VS Code into node-debug.

[isidorn]

August to investigate if we can introduce some vscode api such that the adapter extension could write the launch.json

[isidorn]

For this we are depending on the API for extensions to write to configuration files. Currently in discussion with @bpasero and @jrieken since optimally this would be the same API extensions use to change the user / workspace configuration.
Will probably be pushed out to september.

[weinand]

@isidorn this feature request is about the initial configuration and not about modifying the launch.json at any time. I'm not sure that this feature has to go through the extension API for write to configuration files.

[isidorn]

@weinand somehow github did not notify me about this comment, thus the slow response - sorry.
Yes, but this feature request is a subset of writing to the launch.json at any time - which feels to me like a clean and elegant solution which conforms to our VSCode api.