feat: Add --gitignore flag to read in .gitignore files

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Updates @eslint/migrate-config to use the new gitignore API added in #47

What changes did you make? (Give an overview)

• When --gitignore is passed as an argument, a code block is added to use includeIgnoreFile().
• Updated README
• Added tests

Related Issues

refs #44

Is there anything you'd like reviewers to focus on?

[mdjermanovic]

I tried this with a simple .eslintrc.json example:

{
    "rules": {
        "no-unused-vars": 2
    }
}

eslint.config.cjs result:

const {
    includeIgnoreFile,
} = require("@eslint/compat");

const gitignorePath = path.resolve(__dirname, ".gitignore");

module.exports = [includeIgnoreFile(gitignorePath), {
    rules: {
        "no-unused-vars": 2,
    },
}];

eslint.config.mjs result:

import { includeIgnoreFile } from "@eslint/compat";
const gitignorePath = path.resolve(__dirname, ".gitignore");

export default [includeIgnoreFile(gitignorePath), {
    rules: {
        "no-unused-vars": 2,
    },
}];

In the cjs config, importing path is missing. In the mjs config, importing path and calculating __dirname are missing.

[nzakas]

Ah good catch! I'll fix that.

[mdjermanovic]

LGTM, thanks!