FE-7563 - Fix Accessibility Issues with Mentions Plugin

[nineteen88]

Proposed behaviour

• Resolve Accessibility issues for the Mentions Plugin
• Add design tokens to the Mentions Plugin

Current behaviour

• Axe is showing multiple Accessibility issues with the Mentions plugin when Best Practices is enabled or disabled
• Mentions plugin is using hardcoded styles with no design tokens

Checklist

• Commits follow our style guide
• Related issues linked in commit messages if required
• Screenshots are included in the PR if useful
• All themes are supported if required
• Unit tests added or updated if required
• Playwright automation tests added or updated if required
• Storybook added or updated if required
• Translations added or updated (including creating or amending translation keys table in storybook) if required
• Typescript d.ts file added or updated if required
• Related docs have been updated if required

QA

• Tested in provided StackBlitz sandbox/Storybook
• Add new Playwright test coverage if required
• Carbon implementation matches Design System/designs
• UI Tests GitHub check reviewed if required

Additional context

Testing instructions