Add eslint rule of import/extensions

[taketo1113]

Summary

This Pull Request adds a lint rule to enforce that file extensions(.js) are specified in import statements.

Details

Specifying file extensions is required because, in ESM imports, omitting the file extension(.js) will result in an error.

// correct
import foo from './foo.js';

// error
import foo from './foo';

For more details about the rule being added, see the following:

• eslint-plugin-import: import/extensions
  • https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/extensions.md