```ts
/**
 * The KEY=VALUE pairs of a config file.
 */
export function parseConfig(text: string): Record<string, string> {
  const config: Record<string, string> = {};
  for (const line of text.split("\n")) {
    if (line.trim() === "" || line.startsWith("#")) continue;
    const eqIndex = line.indexOf("=");
    if (eqIndex === -1) throw new Error(`Line '${line}' has no '='`);
    const key = line.substring(0, eqIndex).trim();
    if (key === "__proto__") throw new Error('Invalid key: "__proto__"');
    if (key.trim() === "") throw new Error("Empty key");
    config[key] = line.substring(eqIndex + 1).trim();
  }
  return config;
}
```