test: add more test cases for isPackageJson function (#761)

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## PR Checklist

-   [ ] Addresses an existing open issue: fixes #000
- [ ] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/eslint-plugin-package-json/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [ ] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/eslint-plugin-package-json/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

<!-- Description of what is changed and how the code change does that.
-->

I had developed some vscode extensions. I will add some json files like
`package.nls.json` or `package.nls.zh-cn.json` for internationalization
support. So I add more test cases.


![screenshot of file list with variations of package.json files circled](https://github.com/user-attachments/assets/b1c425f8-3c38-4899-8dfb-2f1fcc1d3f55)

src/utils/isPackageJson.test.ts

@@ -12,6 +12,8 @@ describe("isPackageJson", () => {
 		["-package.json", false],
 		["prefix-package.json", false],
 		["package.json.json", false],
+		["package.nls.json", false],
+		["package.nls.zh-cn.json", false],
 		["package.package.json", false],
 		["package.json.package.json", false],
 		["package.json-package.json", false],