Add $schema to cgmanifest.json (#13887)

This pull request adds the JSON schema for `cgmanifest.json`.

## FAQ

### Why?

A JSON schema helps you to ensure that your `cgmanifest.json` file is valid.
JSON schema validation is a build-in feature in most modern IDEs like Visual Studio and Visual Studio Code.
Most modern IDEs also provide code-completion for JSON schemas.

### How can I validate my `cgmanifest.json` file?

Most modern IDEs like Visual Studio and Visual Studio Code have a built-in feature to validate JSON files.
You can also use [this small script](https://github.com/JamieMagee/verify-cgmanifest) to validate your `cgmanifest.json` file.

### Why does it suggest camel case for the properties?

Component Detection is able to read camel case and pascal case properties.
However, the JSON schema doesn't have a case-insensitive mode.
We therefore suggest camel case as it's the most common format for JSON.

### Why is the diff so large?

To deserialize the `cgmanifest.json` file, we use [`JSON.parse()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse).
However, to serialize the JSON again we use [`prettier`](https://prettier.io/).
We found that, in general, it gave smaller diffs than the default [`JSON.stringify()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify) function.

dep/CLI11/cgmanifest.json

@@ -1,13 +1,15 @@
-{"Registrations":[
+{
+  "$schema": "https://json.schemastore.org/component-detection-manifest.json",
+  "Registrations": [
     {
       "component": {
-       "type": "git",
-       "git": {
-         "repositoryUrl": "https://github.com/CLIUtils/CLI11",
-         "commitHash": "dd0d8e4fe729e5b1110232c7a5c9566dad884686"
-         }
-       }
+        "type": "git",
+        "git": {
+          "repositoryUrl": "https://github.com/CLIUtils/CLI11",
+          "commitHash": "dd0d8e4fe729e5b1110232c7a5c9566dad884686"
+        }
+      }
     }
- ],
- "Version": 1
+  ],
+  "Version": 1
 }

dep/jsoncpp/cgmanifest.json

@@ -1,13 +1,15 @@
-{"Registrations":[
+{
+  "$schema": "https://json.schemastore.org/component-detection-manifest.json",
+  "Registrations": [
     {
       "component": {
-       "type": "git",
-       "git": {
-         "repositoryUrl": "https://github.com/open-source-parsers/jsoncpp",
-         "commitHash": "6aba23f4a8628d599a9ef7fa4811c4ff6e4070e2"
-         }
-       }
+        "type": "git",
+        "git": {
+          "repositoryUrl": "https://github.com/open-source-parsers/jsoncpp",
+          "commitHash": "6aba23f4a8628d599a9ef7fa4811c4ff6e4070e2"
+        }
+      }
     }
- ],
- "Version": 1
+  ],
+  "Version": 1
 }