Add execCommand (#1469)

Close #1468, adding the suggested spec to the list.

### Changes to `index.json`
This update would trigger the following changes in `index.json`:

<details><summary>Add spec (1)</summary>

```json
{
  "url": "https://w3c.github.io/editing/docs/execCommand/",
  "seriesComposition": "full",
  "shortname": "execCommand",
  "series": {
    "shortname": "execCommand",
    "currentSpecification": "execCommand",
    "title": "execCommand",
    "shortTitle": "execCommand",
    "nightlyUrl": "https://w3c.github.io/editing/docs/execCommand/"
  },
  "standing": "pending",
  "nightly": {
    "url": "https://w3c.github.io/editing/docs/execCommand/",
    "status": "Unofficial Proposal Draft",
    "sourcePath": "docs/execCommand/index.html",
    "alternateUrls": [],
    "repository": "https://github.com/w3c/editing",
    "filename": "index.html"
  },
  "organization": "W3C",
  "groups": [
    {
      "name": "Web Editing Working Group",
      "url": "https://www.w3.org/groups/wg/webediting/"
    }
  ],
  "title": "execCommand",
  "source": "spec",
  "shortTitle": "execCommand",
  "categories": [
    "browser"
  ]
}
```
</details>

### Tests
These changes look good! 😎

Co-authored-by: browser-specs-bot <>

specs.json

@@ -489,6 +489,14 @@
   "https://urlpattern.spec.whatwg.org/",
   "https://w3c.github.io/audio-session/",
   "https://w3c.github.io/contentEditable/",
+  {
+    "shortname": "execCommand",
+    "standing": "pending",
+    "nightly": {
+      "sourcePath": "docs/execCommand/index.html"
+    },
+    "url": "https://w3c.github.io/editing/docs/execCommand/"
+  },
   "https://w3c.github.io/gamepad/extensions.html",
   "https://w3c.github.io/long-animation-frames/",
   "https://w3c.github.io/mathml-aam/",

src/data/monitor.json

@@ -418,10 +418,6 @@
       "comment": "being split-off from main spec https://github.com/w3c/PFE/pull/9/files; stub",
       "lastreviewed": "2024-03-01"
     },
-    "https://w3c.github.io/editing/docs/execCommand/": {
-      "lastreviewed": "2024-03-01",
-      "comment": "no longer worked on and not implemented consistently or fully by user agents"
-    },
     "https://dev.w3.org/html5/html-sourcing-inband-tracks/": {
       "lastreviewed": "2024-03-01",
       "comment": "no longer worked on and implementation status is unclear"