feat(require-files): add require-files rule (#887)

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## PR Checklist

-   [x] Addresses an existing open issue: fixes #803 
- [x] 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)
- [x] 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.
-->

Add `require-files` rule

README.md

@@ -127,6 +127,7 @@ The default settings don't conflict, and Prettier plugins can quickly fix up ord
 | [order-properties](docs/rules/order-properties.md)                     | Package properties must be declared in standard order                                             | ✅  | 🔧 |    |    |
 | [repository-shorthand](docs/rules/repository-shorthand.md)             | Enforce either object or shorthand declaration for repository.                                    | ✅  | 🔧 |    |    |
 | [require-author](docs/rules/require-author.md)                         | Requires the `author` property to be present.                                                     |    |    |    |    |
+| [require-files](docs/rules/require-files.md)                           | Requires the `files` property to be present.                                                      |    |    |    |    |
 | [require-keywords](docs/rules/require-keywords.md)                     | Requires the `keywords` property to be present.                                                   |    |    |    |    |
 | [require-name](docs/rules/require-name.md)                             | Requires the `name` property to be present.                                                       | ✅  |    |    |    |
 | [require-version](docs/rules/require-version.md)                       | Requires the `version` property to be present.                                                    | ✅  |    |    |    |

docs/rules/require-files.md

@@ -0,0 +1,25 @@
+# require-files
+
+<!-- end auto-generated rule header -->
+
+This rule checks for the existence of the `"files"` property in a package.json,
+and reports a violation if it doesn't exist.
+
+Example of **incorrect** code for this rule:
+
+```json
+{
+	"name": "thee-silver-mt-zion",
+	"version": "13.0.0"
+}
+```
+
+Example of **correct** code for this rule:
+
+```json
+{
+	"name": "thee-silver-mt-zion",
+	"version": "13.0.0",
+	"files": ["lib"]
+}
+```

src/rules/require-properties.ts

@@ -6,6 +6,7 @@ import { createRequirePropertyRule } from "../utils/createRequirePropertyRule.js
 // in the format [propertyName, isRecommended]
 const properties = [
 	["author", false],
+	["files", false],
 	["keywords", false],
 	["name", true],
 	["version", true],

src/tests/rules/require-files.test.ts

@@ -0,0 +1,47 @@
+import { rules } from "../../rules/require-properties.js";
+import { ruleTester } from "./ruleTester.js";
+
+ruleTester.run("require-files", rules["require-files"], {
+	invalid: [
+		{
+			code: "{}",
+			errors: [
+				{
+					data: { property: "files" },
+					line: 1,
+					messageId: "missing",
+				},
+			],
+		},
+		{
+			code: `{
+            "version": "1.0.0"
+            }
+            `,
+			errors: [
+				{
+					data: { property: "files" },
+					line: 1,
+					messageId: "missing",
+				},
+			],
+		},
+		{
+			code: `{
+            "author": "Jessica Moss",
+            "bin": {
+                "files": ["lib"]
+            }
+            }
+            `,
+			errors: [
+				{
+					data: { property: "files" },
+					line: 1,
+					messageId: "missing",
+				},
+			],
+		},
+	],
+	valid: [`{ "files": ["lib"] }`],
+});