Add sourcemap support

[timneutkens]

Tested in vscode's debug mode (which is excellent btw) with this configuration:

{
    // Use IntelliSense to learn about possible Node.js debug attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "type": "node2",
            "request": "launch",
            "name": "Launch Program",
            "program": "${workspaceRoot}/node_modules/micro/bin/micro",
            "cwd": "${workspaceRoot}",
            "args": ["."],
            "sourceMaps": true,
            "env": {
                "NODE_ENV": "development"
            }
        },
        {
            "type": "node2",
            "request": "attach",
            "name": "Attach to Process",
            "port": 5858
        }
    ]
}

[timneutkens]

@jaredpalmer 🙌

[timneutkens]

Closing #90 after this is merged.