fix(decimal): preserve leading positive sign in formatted value

[tomdavies73]

fix #7682

Proposed behaviour

A leading positive sign is not removed when the inputted value is formatted

Current behaviour

A leading positive sign is removed when the inputted value is formatted

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

1. Find the Default story locally and in sage.carbon
2. Locally add a + to the input then blur the input (tab or click away)
3. On sage.carbon add a + to the input then blur the input (tab or click away)
4. Verify locally the + remains (correct behaviour fixed here) and on sage.carbon the + is removed (incorrect behaviour which is addressed as part of this PR)