Support java debugging

The java architecture is a little different:

- the debugger is a jdt.ls extension bundle.
- to start the server, you send a startDebugSession command to jdt.ls
- this returns a tcl port to connect to for DAP

Loading of jdt.ls and the extension are out of scope for vimspector
currently and instead you can tell it to ask you for a port to connect
to.

After connexting to that port, vimspector works as normal.

To support TCP/IP was pretty simple: we provide the same API from
vimscript as the job-based (stdin/out) comms layer, but instead just
directly use a channel.

The only wrinkle was that the java debug adapter broke the protocol on
runInTerminal and didn't return a 'cwd', so we make one up.

support/test/java/test_project/.vimspector.json

@@ -0,0 +1,22 @@
+{
+  "adapters": {
+    "java-debug-server": {
+      "name": "vscode-java",
+      "port": "ask"
+    }
+  },
+  "configurations": {
+    "Java Launch": {
+      "adapter": "java-debug-server",
+      "configuration": {
+        "request": "launch",
+        "mainClass": "com.vimspector.test.TestApplication",
+        "sourcePaths": [ "${workspaceRoot}/src/main/java" ],
+        "classPaths": [ "${workspaceRoot}/target/classes" ],
+        "args": "hello world!",
+        "stopOnEntry": true,
+        "console": "integratedTerminal"
+      }
+    }
+  }
+}