feat: fill in existing contributors in hydration (#639)

## PR Checklist

- [x] Addresses an existing open issue: fixes #448
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/template-typescript-node-package/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/template-typescript-node-package/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Adds detecting existing contributors with
[`all-contributors-for-repository`](https://github.com/JoshuaKGoldberg/all-contributors-for-repository),
along with a `--skip-contributors` flag to skip that.

Author: JoshuaKGoldberg

README.md

@@ -129,6 +129,7 @@ npx template-typescript-node-package --releases false --unitTests false
 
 You can prevent the hydration script from making network-based changes using either or both of the following CLI flags:
 
+- `--skip-contributors` _(`boolean`)_: Skips detecting existing contributors with [`all-contributors-for-repository`](https://github.com/JoshuaKGoldberg/all-contributors-for-repository)
 - `--skip-install` _(`boolean`)_: Skips installing all the new template packages with `pnpm`
 - `--skip-setup` _(`boolean`)_: Skips running the setup script at the end of hydration
 

package.json

@@ -43,6 +43,7 @@
 	},
 	"dependencies": {
 		"@clack/prompts": "^0.6.3",
+		"all-contributors-for-repository": "^0.0.10",
 		"chalk": "^5.2.0",
 		"execa": "^7.1.1",
 		"git-remote-origin-url": "^4.0.0",