Improve "Generate launch configuration" user experience

"Generate launch configuration" is extremely useful, but the generated configuration is missing several items I consider to be necessary for smooth debugging in vscode:
program should be relative to ${workspaceFolder} if possible. Can we leave off the .exe in program? If so we should forward slash for Windows and strip the exe. The idea here is to make it easier to share a given debug configuration with others, even if you don't check in your launch.json.

"preLaunchTask": "rust: cargo build",

should be specified to ensure that we are built before launching. The exact task should be changed to whatever task is most likely to actually build the target we are debugging (for example, test targets probably need a different task).

"internalConsoleOptions": "openOnSessionStart",

ensures that the debug pane opens on debug session start (should be vscode default imo).

env contains far too many keys by default. I don't know if there is a good way of fixing this though.

No file or test is named in the issue. Start by locating the code that generates VS Code launch configurations, then compare its output with the requested program path, build task, console setting, and environment handling. Done means the generated launch.json is easier to share and supports the intended build-and-debug flow.