Create test for contractions

Problem to solve

The GitLab style guide (https://docs.gitlab.com/ee/development/documentation/styleguide.html) encourages the use of some contractions, but not others, and does not alert the TW team when they are working on a page that violates the style guide

Further details

Proposal

Create a test in Vale that allows the acceptable contractions (https://docs.gitlab.com/ee/development/documentation/styleguide.html#language) but not the disallowed ones.

Who can address the issue

@aqualls or @mjang1.

Other links/references

Part of &2497 (closed).