Provide node API for config conversion logic

[JamesHenry]

🚀 Feature Request

Existing Behavior

The library can be leveraged via a CLI and interacts directly with disk.

Change Proposal

Provide the option of leveraging (part of) the library in code via a node API which would only focus on the config conversion process.

After discussing this with @JoshuaKGoldberg it seems we essentially want to expose (potentially after some tweaks) the convertLintConfig function:

tslint-to-eslint-config/src/converters/lintConfigs/convertLintConfig.ts

Line 22 in e425bab

export const convertLintConfig = async (

[JoshuaKGoldberg]

Just posting notes from our chat: this is definitely doable and something we'd want to expose. There are actually quite a few pieces that go into converting to an output ESLint config:

• Original TSLint rules, both as the raw tslint.json contents and the result of tslint --print-config
• Original ESLint rules, if a config already exists (originalConfigurations.eslint)
• TypeScript settings (tsconfig.json)
• Node package contents (package.json)

Out of those, only the (reported?) tslint.json ones are really necessary. I don’t quite remember off the top of my head why both are there, but IIRC there was a reason. More investigation required 😄