fix: add bin key back to package.json (#101)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #100
- [ ] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/package-json-validator/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
   - I just opened this Issue, and then added a PR to address it.
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/package-json-validator/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

<!-- Description of what is changed and how the code change does that.
-->
Re-added the `bin` key to package.json so that `npx
package-json-validator` works without having to install the package.

## Output

```
$ npx package-json-validator --warnings --recommendations
{ valid: true }
```

package.json

@@ -35,6 +35,9 @@
 	],
 	"type": "commonjs",
 	"main": "PJV.js",
+	"bin": {
+		"pjv": "./bin/pjv"
+	},
 	"files": [
 		"bin/",
 		"demo/",