Normalize code files

As I work in this codebase, I get tons of diff noise from trailing whitespace and missing newlines at the ends of files that PhpStorm/VSCode strip out and generative AI can't seem to help itself from fixing when making unrelated changes. This issue is to fix all those issues once and for all. In all code files, it...

• Removes trailing whitespace
• Ensures exactly one newline character at the end
• Updates .gitattributes with a change that ddev/ddev-drupal-contrib makes every time you update Composer dependencies

It's kind of a long diff, but it's nothing but whitespace changes except .gitattributes, and even that's not a functional change. I asked GitHub Copilot to examine the diff, and it said, "there is nothing but whitespace changes in this diff. There are no changes that warrant manual testing or a closer look."