Fix missing GitHub Sponsor for npm fund (#7707)

We forgot to add our GitHub Sponsor to `package.json` when we started it.

```sh-session
$ npm fund stylelint
1: opencollective funding available at the following URL: https://opencollective.com/stylelint
2: github funding available at the following URL: https://github.com/sponsors/stylelint
Run `npm fund [<package-spec>] --which=1`, for example, to open the first funding URL listed in that package
```

In addition, this improves the financial section in the contribution guide:

- Add GitHub Sponsor
- Simplify links to Open Collective, including unsupported `#backer` or `#sponsor`

.changeset/tricky-pigs-talk.md

@@ -0,0 +1,5 @@
+---
+"stylelint": patch
+---
+
+Fixed: missing GitHub Sponsor for `npm fund`

CONTRIBUTING.md

@@ -67,11 +67,8 @@ After we review and merge your pull request, we'll invite you to become a mainta
 
 ## Financial contributions
 
-We welcome financial contributions in full transparency on our [Open Collective](https://opencollective.com/stylelint).
+We welcome financial contributions in full transparency on our [Open Collective](https://opencollective.com/stylelint) or [GitHub Sponsor](https://github.com/sponsors/stylelint).
 
 Anyone can file an expense. We will "merge" the expense into the ledger if it makes sense for the development of the community. Open Collective then reimburses the person who filed the expense.
 
-You can financially support us by becoming a:
-
-- [backer](https://opencollective.com/stylelint#backer)
-- [sponsor](https://opencollective.com/stylelint#sponsor)
+You can financially support us by becoming a [backer or sponsor on Open Collective](https://opencollective.com/stylelint) or a [sponsor on GitHub](https://github.com/sponsors/stylelint).

package.json

@@ -16,10 +16,16 @@
   ],
   "homepage": "https://stylelint.io",
   "repository": "stylelint/stylelint",
-  "funding": {
-    "type": "opencollective",
-    "url": "https://opencollective.com/stylelint"
-  },
+  "funding": [
+    {
+      "type": "opencollective",
+      "url": "https://opencollective.com/stylelint"
+    },
+    {
+      "type": "github",
+      "url": "https://github.com/sponsors/stylelint"
+    }
+  ],
   "license": "MIT",
   "author": "stylelint",
   "exports": {