debug: implement new prototype thin DA for delve

Updates #23

This DA serves as a DAP proxy between the editor and a DAP-capable Delve. Right now all it does is launch Delve (in DAP mode) when LaunchRequest is received from the editor, and then relays all messages between the editor and Delve back and forth.

package.json section is copied from the current DA as-is. Some of its options will be removed and cleaned up in the subsequent PRs to make the diffs more obvious (see #271).

Change-Id: I3493c5ee1d9e80071a17ea32c04a15eb021c8006
GitHub-Last-Rev: ddd68dc
GitHub-Pull-Request: #267
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/240417
Reviewed-by: Polina Sokolova <polina@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

.vscode/launch.json

@@ -34,6 +34,21 @@
 			],
 			"preLaunchTask": "npm"
 		},
+		{
+			"name": "Launch as server (dlv dap)",
+			"type": "node",
+			"protocol": "inspector",
+			"request": "launch",
+			"program": "${workspaceFolder}/out/src/debugAdapter2/goDlvDebugMain.js",
+			"args": [
+				"--server=4711"
+			],
+			"sourceMaps": true,
+			"outFiles": [
+				"${workspaceFolder}/out/**/*.js"
+			],
+			"preLaunchTask": "npm"
+		},
 		{
 			"name": "Launch Extension Tests",
 			"type": "extensionHost",