feat(responsive-vertical-menu): allows custom aria-label for menu launcher

[damienrobson-sage]

Proposed behaviour

• ResponsiveVerticalMenu now allows an additional prop to set the value of aria-label;
• The ResponsiveVerticalMenuDivider is now exported correctly

Current behaviour

• The launcher button for the ResponsiveVerticalMenu is announced via the name of the icon used, with no additional context;
• The ResponsiveVerticalMenuDivider isn't exported for use

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

Testing instructions

Use the Without Global Header test story to verify that Main menu is announced when focus is gained and a screen reader is active

[edleeks87]

I think it's worth doing the same for this before we send it off to the translation team