Exclude bootstrap contrib code from automated code style checking

Problem/Motivation

The automated tests on Drupal.org infrastructure include code style validation using phpcs. This check reports the bootstrap contributed code as code style violations.

Proposed resolution

Exclude bootstrap contrib code from automated code style checking using a custom phpcs.xml file.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.