Enable embedded vim python debugging

This allows us to use vimspector to debug vimspector. The idea is that
in Vim you call 'py3 from vimspector.developer import SetUpDebugpy;
SetUpDebugpy()' and then just attach to localhost:5678 using the new
multi-session mode support. Oof.

.vimspector.json

@@ -0,0 +1,14 @@
+{
+  "configurations": {
+    "Python: Attach To Vim": {
+      "variables": {
+        "port": "5678",
+        "host": "localhost"
+      },
+      "adapter": "multi-session",
+      "configuration": {
+        "request": "attach"
+      }
+    }
+  }
+}

python3/vimspector/developer.py

@@ -0,0 +1,42 @@
+# vimspector - A multi-language debugging system for Vim
+# Copyright 2020 Ben Jackson
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+import sys
+import os
+
+from vimspector import install, utils
+
+
+def SetUpDebugpy( wait=False, port=5678 ):
+  sys.path.insert(
+    1,
+    os.path.join( install.GetGadgetDir( utils.GetVimspectorBase(),
+                                        install.GetOS() ),
+                  'debugpy',
+                  'build',
+                  'lib' ) )
+  import debugpy
+
+  exe = sys.executable
+  try:
+    # debugpy uses sys.executable (which is `vim`, so we hack it)
+    sys.executable = 'python3'
+    debugpy.listen( port )
+  finally:
+    sys.executable = exe
+
+  if wait:
+    debugpy.wait_for_client()