Refactor build/config/keymaps.

* Move all headers to `include/zmk` directory.
* Update includes to reference them properly.
* Add `keymap.c` file from keymap, if found.
* Toplevel CONFIG_ZMK_BLE aggregate config setting.

.vscode/tasks.json

@@ -0,0 +1,28 @@
+{
+  // See https://go.microsoft.com/fwlink/?LinkId=733558
+  // for the documentation about the tasks.json format
+  "version": "2.0.0",
+  "tasks": [
+    {
+      "label": "Build",
+      "type": "shell",
+      "command": "west build",
+      "group": {
+        "kind": "build",
+        "isDefault": true
+      }
+    },
+    {
+      "label": "Flash",
+      "type": "shell",
+      "command": "west flash",
+      "group": "test"
+    },
+    {
+      "label": "Debug",
+      "type": "shell",
+      "command": "west debug",
+      "group": "test"
+    }
+  ]
+}