chore(fieldset): add next internal component

[nuria1110]

Proposed behaviour

Adds new internal fieldset component to be used in CheckboxGroup, RadioButtonGroup, NumeralDate and Time components once they are aligned with the DS. This allows for components to be updated separately, avoids duplication and ensures consistency across components.

Current behaviour

Internal Fieldset component contains legacy props and styles.

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