fix(namespaces): only report when there is value emit (#27)

## PR Checklist

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

## Overview

Changes the rule to not report when no value emit child is found in a
namespace.

Adds [`cached-factory`](https://www.npmjs.com/package/cached-factory) as
a dependency to cache checks on individual namespace nodes. Parent
namespaces need to check their child namespaces.

💖

Author: JoshuaKGoldberg

cspell.json

@@ -14,6 +14,7 @@
 		"automerge",
 		"joshuakgoldberg",
 		"markdownlintignore",
-		"tseslint"
+		"tseslint",
+		"TSESTree"
 	]
 }

docs/rules/namespaces.md

@@ -4,6 +4,8 @@
 
 Enforces that code doesn't use TypeScript's `namespaces` with values:
 
+## Invalid Code
+
 ```ts
 module Values {
 	export const value = "a";
@@ -13,3 +15,15 @@ namespace Values {
 	export const value = "a";
 }
 ```
+
+## Valid Code
+
+```ts
+module Values {
+	export type Value = "a";
+}
+
+namespace Values {
+	export type Value = "a";
+}
+```

package.json

@@ -37,7 +37,8 @@
 		"*": "prettier --ignore-unknown --write"
 	},
 	"dependencies": {
-		"@typescript-eslint/utils": "^8.24.0"
+		"@typescript-eslint/utils": "^8.24.0",
+		"cached-factory": "^0.1.0"
 	},
 	"devDependencies": {
 		"@eslint-community/eslint-plugin-eslint-comments": "4.4.1",