Correct keyboard shortcuts commands for headings

The proposed changes were based on @jasongrout's suggestions in issue #4428.

In `jupyterlab/packages/shortcuts-extension/schema/plugin.json`, keyboard shortcut `1-6` are registered with commands `change-`**`to-cell`**`-heading-X`, whereas the commands in `jupyterlab/packages/notebook-extension/src/index.ts` are named `change-`**`cell-to`**`-heading-X`. As a result, the keyboard shortcuts are not working. 

This PR attempts to fix this problem by changing the command names in `jupyterlab/packages/shortcuts-extension/schema/plugin.json` to the ones in `jupyterlab/packages/notebook-extension/src/index.ts`.

packages/shortcuts-extension/schema/plugin.json

@@ -256,55 +256,55 @@
       },
       "type": "object"
     },
-    "notebook:change-to-cell-heading-1": {
+    "notebook:change-cell-to-heading-1": {
       "default": { },
       "properties": {
-        "command": { "default": "notebook:change-to-cell-heading-1" },
+        "command": { "default": "notebook:change-cell-to-heading-1" },
         "keys": { "default": ["1"] },
         "selector": { "default": ".jp-Notebook:focus" }
       },
       "type": "object"
     },
-    "notebook:change-to-cell-heading-2": {
+    "notebook:change-cell-to-heading-2": {
       "default": { },
       "properties": {
-        "command": { "default": "notebook:change-to-cell-heading-2" },
+        "command": { "default": "notebook:change-cell-to-heading-2" },
         "keys": { "default": ["2"] },
         "selector": { "default": ".jp-Notebook:focus" }
       },
       "type": "object"
     },
-    "notebook:change-to-cell-heading-3": {
+    "notebook:change-cell-to-heading-3": {
       "default": { },
       "properties": {
-        "command": { "default": "notebook:change-to-cell-heading-3" },
+        "command": { "default": "notebook:change-cell-to-heading-3" },
         "keys": { "default": ["3"] },
         "selector": { "default": ".jp-Notebook:focus" }
       },
       "type": "object"
     },
-    "notebook:change-to-cell-heading-4": {
+    "notebook:change-cell-to-heading-4": {
       "default": { },
       "properties": {
-        "command": { "default": "notebook:change-to-cell-heading-4" },
+        "command": { "default": "notebook:change-cell-to-heading-4" },
         "keys": { "default": ["4"] },
         "selector": { "default": ".jp-Notebook:focus" }
       },
       "type": "object"
     },
-    "notebook:change-to-cell-heading-5": {
+    "notebook:change-cell-to-heading-5": {
       "default": { },
       "properties": {
-        "command": { "default": "notebook:change-to-cell-heading-5" },
+        "command": { "default": "notebook:change-cell-to-heading-5" },
         "keys": { "default": ["5"] },
         "selector": { "default": ".jp-Notebook:focus" }
       },
       "type": "object"
     },
-    "notebook:change-to-cell-heading-6": {
+    "notebook:change-cell-to-heading-6": {
       "default": { },
       "properties": {
-        "command": { "default": "notebook:change-to-cell-heading-6" },
+        "command": { "default": "notebook:change-cell-to-heading-6" },
         "keys": { "default": ["6"] },
         "selector": { "default": ".jp-Notebook:focus" }
       },