doc: add custom module example

Author: sebastianwessel

example/custom-module/README.md

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+# Using Custom Node Module
+
+This is a basic example of how to use [@sebastianwessel/quickjs](https://github.com/sebastianwessel/quickjs). You can test it out by running:
+
+```sh
+bun run example:module
+```
+
+from the root of this repository.

example/custom-module/custom.js

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+export const customFn = () => {
+	return 'Hello from the custom module'
+}

example/custom-module/index.ts

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+import { join } from 'node:path'
+import { dirname } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { quickJS } from '@sebastianwessel/quickjs'
+
+// General setup like loading and init of the QuickJS wasm
+// It is a ressource intensive job and should be done only once if possible
+const { createRuntime } = await quickJS()
+
+const __dirname = dirname(fileURLToPath(import.meta.url))
+const customModuleHostLocation = join(__dirname, './custom.js')
+
+// Create a runtime instance each time a js code should be executed
+const { evalCode } = await createRuntime({
+	nodeModules: {
+		// module name
+		'custom-module': {
+			// key must be index.js, value file content of module
+			'index.js': await Bun.file(customModuleHostLocation).text(),
+		},
+	},
+})
+
+const result = await evalCode(`
+import { customFn } from 'custom-module'
+
+const result = customFn()
+
+console.log(result)
+  
+export default result
+`)
+
+console.log(result) // { ok: true, data: 'Hello from the custom module' }