Use MONO_ENV_OPTIONS to pass the debug arguments to mono (#66)

This allows passing the arguments to e.g. a mono built from the dotnet/runtime repo since the executable doesn't take the usual mono options.

Example of launch.json:

```
{
            "type": "mono",
            "request": "launch",
            "name": "mono-netcore",
            "program": "/Users/thaysgrazia/runtime/artifacts/bin/MyConsoleApp/Debug/net5.0/MyConsoleApp.dll",
            "runtimeExecutable": "/Users/thaysgrazia/runtime/.dotnet-mono/dotnet"
}
```

Co-authored-by: Alexander Köplinger <alex.koeplinger@outlook.com>

src/MonoDebugSession.cs

@@ -232,16 +232,12 @@ public override async void Launch(Response response, dynamic args)
 
 
 			// validate argument 'env'
-			Dictionary<string, string> env = null;
+			Dictionary<string, string> env = new Dictionary<string, string>();
 			var environmentVariables = args.env;
 			if (environmentVariables != null) {
-				env = new Dictionary<string, string>();
 				foreach (var entry in environmentVariables) {
 					env.Add((string)entry.Name, (string)entry.Value);
 				}
-				if (env.Count == 0) {
-					env = null;
-				}
 			}
 
 			const string host = "127.0.0.1";
@@ -260,11 +256,18 @@ public override async void Launch(Response response, dynamic args)
 			var cmdLine = new List<String>();
 
 			bool debug = !getBool(args, "noDebug", false);
+
 			if (debug) {
-				cmdLine.Add("--debug");
-				cmdLine.Add(String.Format("--debugger-agent=transport=dt_socket,server=y,address={0}:{1}", host, port));
+				if (!env.ContainsKey("MONO_ENV_OPTIONS"))
+					env["MONO_ENV_OPTIONS"] = $" --debug --debugger-agent=transport=dt_socket,server=y,address={host}:{port}";
+				else
+					env["MONO_ENV_OPTIONS"] = $" --debug --debugger-agent=transport=dt_socket,server=y,address={host}:{port} " + env["MONO_ENV_OPTIONS"];
 			}
-
+
+			if (env.Count == 0) {
+				env = null;
+			}
+
 			// add 'runtimeArgs'
 			if (args.runtimeArgs != null) {
 				string[] runtimeArguments = args.runtimeArgs.ToObject<string[]>();
@@ -305,13 +308,12 @@ public override async void Launch(Response response, dynamic args)
 			if (console == "externalTerminal" || console == "integratedTerminal") {
 
 				cmdLine.Insert(0, mono_path);
-
 				var termArgs = new {
 					kind = console == "integratedTerminal" ? "integrated" : "external",
 					title = "Node Debug Console",
 					cwd = workingDirectory,
 					args = cmdLine.ToArray(),
-					env = environmentVariables
+					env
 				};
 
 				var resp = await SendRequest("runInTerminal", termArgs);