Align config basePath with ESLint flat config

[chenjiahan]

Summary

Rslint should consider supporting the ESLint flat config basePath option on config entries.

In ESLint, basePath sets a subdirectory base for a config object. Relative basePath values are resolved from the config file directory, or from cwd when using an alternate config via --config; files and ignores in that object are then resolved relative to that base path.

Why

Rslint recently aligned explicit --config handling with ESLint by resolving files, ignores, and parserOptions.project from cwd. Supporting basePath would close the next compatibility gap for users sharing or migrating ESLint flat configs.

Without support, configs like this are silently interpreted with the wrong match base:

export default [
  {
    basePath: 'packages/foo',
    files: ['src/**/*.ts'],
    ignores: ['fixtures/**'],
  },
];

Suggested behavior

• Add basePath?: string to config entries.
• Resolve relative basePath from the config directory for auto-discovered configs.
• Resolve relative basePath from cwd for explicit --config configs.
• Resolve that entry's files, ignores, and languageOptions.parserOptions.project relative to the effective base path.
• Treat { basePath, ignores } as a global ignore entry, matching ESLint's intent for scoped global ignores.

References

• ESLint docs: https://eslint.org/docs/latest/use/configure/configuration-files#specify-files-and-ignores
• ESLint issue: Change Request: Config base path in flat config eslint/eslint#15683