Add PR template to design tokens repo

[georgewrmarshall]

Description

We need to create a PR template that can be used for all design token PRs and aligns with mobile and extension codebases.

• Mobile PR template
• Extension PR template

Technical Details

• Add .github/PULL_REQUEST_TEMPLATE.md that aligns with mobile and the extension

Acceptance Criteria

• The PR template should include sections for: Description, Issue #, Testing Steps, Screenshots, Design Tokens (if applicable), and Additional Information.
• The PR template should specifically ask for testing steps and screenshots in both mobile and extension environments if the PR involves changes to the design tokens.
• The PR template should be usable in both the mobile and extension repositories.

Additional Information

Once the PR template is created, it should be added to the .github directory in both the mobile and extension

[georgewrmarshall]

Closed by #601 here 5210fff