Auto-generate launch.json tasks

[rockbruno]

Now that we're able to compile targets individually, we should be able to do something similar to the Swift extension in terms of auto-generating build/launch tasks for each library parsed by the BSP.

I'm thinking something like this:

• New flag: generate_launch_json (bool)
• New flag: targets_to_generate_launch_data ([label])
  • If empty, generate entries for all parsed targets. In practice large projects will probably want to be explicit here.
• After parsing the target list in BazelTargetStore, append/override the BSP's data in the file. The Swift extension does exactly that so we should be able to find some relevant code there.

[aaronsky]

one thing to consider is that, afaik, launch.json is explicitly a VSCode construct, whereas the BSP and its support so far have been suitably editor-agnostic

[rockbruno]

Good point, we might want to somehow separate that in this case.