Resolves #10614
Resolves WOOPMNT-4876

Changes proposed in this Pull Request

This PR adds support for Singapore-based merchants to provide their tax ID numbers. It implements UI changes to accommodate both UEN (Unique Entity Number) and GST Registration Number formats.

Key Changes:

• Added Singapore-specific tax ID prefix handling (empty prefix for SG)
• Updated tax ID field label to "UEN or GST Registration Number" for SG merchants
• Added validation hint showing example formats for both UEN and GST numbers
• Implemented required self-declaration message for GST registered businesses
• Follows specifications from IRAS, WooPayments VAT Matrix and Taxamo

Testing instructions

• Run npm run changelog to add a changelog file, choose patch to leave it empty if the change is not significant. You can add multiple changelog files in one PR by running this command a few times.
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Link to testing instructions from release testing doc following these instructions : Add link here / 'QA Testing Not Applicable'
• Add or update critical flows and testing instructions for critical flows, if applicable.
• Add what's changed (description, screenshot, demo videos etc.) to the release announcement post, if applicable.