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/.gitignore

@@ -0,0 +1,4 @@
+.classpath
+.project
+.settings/
+target/