Running twice adds extended configuration's values in output config

[JoshuaKGoldberg]

🐛 Bug Report

• tslint-to-eslint-config version: 1.0.0-beta
• ESLint version: N/A
• Node version: N/A

Actual Behavior

When running tslint-to-eslint-config in a directory that already has an ESLint configuration file, if the file extends from an ESLint configuration, rules from the configuration are getting added to the config's rule list.

    "rules": {
        "arrow-body-style": [
            0
        ],
        "curly": [
            0
        ],
        ...

Expected Behavior

When an ESLint exists already and extends from an existing ESLint configuration, it shouldn't feel the need to manually redeclare the rules' arguments and severities.

Reproduction

1. Create a new directory with npm init
2. Create a tslint.json with just {} as its contents
3. npx tslint-to-eslint-config -> configuration looks fine
4. npx tslint-to-eslint-config -> configuration now manually lists out ESLint rule values...