Add a standard for importing traits

Problem/Motivation

There's no coding standard for traits AFAICT.

Some questions:

- should the 'use FooTrait;' statements be sorted? and how: alphabetically?
- should there be a blank line or not between the class declaration and the trait import? I've seen both in core:

class Foo {
  use MyTrait;

or

class Foo {

  use MyTrait;

Benefits

If we adopted this change, the Drupal Project would benefit by ...

Three supporters required

1. https://www.drupal.org/u/ → {userid} (yyyy-mm-dd they added support)
2. https://www.drupal.org/u/ → {userid} (yyyy-mm-dd they added support)
3. https://www.drupal.org/u/ → {userid} (yyyy-mm-dd they added support)

Proposed changes

Provide all proposed changes to the Drupal Coding standards → . Give a link to each section that will be changed, and show the current text and proposed text as in the following layout:

1. PHP coding standards →

Add current text in blockquotes

Add proposed text in blockquotes

2. Repeat the above for each page or sub-page that needs to be changed.

Remaining tasks

2. Add supporters
3. Create a Change Record
4. Review by the Coding Standards Committee
5. Coding Standards Committee takes action as required
6. Discussed by the Core Committer Committee, if it impacts Drupal Core
7. Final review by Coding Standards Committee
8. Documentation updates
   1. Edit all pages
   2. Publish change record
   3. Remove 'Needs documentation edits' tag
9. If applicable, create follow-up issues for PHPCS rules/sniffs changes

For a full explanation of these steps see the Coding Standards project page →