[Analytics Hub] Add helpers for creating the delta percentage text

[rachelmcr]

Part of: #8149
⚠️ Depends on #8175 ⚠️

Description

This PR adds a set of helpers for generate the delta percentage text (e.g. +36% or -16%) for Analytics Hub cards.

Changes

• Adds general helpers and unit tests for creating the delta percentage text from two values (Decimal or Double types).
• Adds specific helpers and unit tests for creating the delta percentage text for specific stats used in Analytics Hub, e.g. Total Orders, Average Order Value.

Testing

These helpers aren't yet used in the app; confirm the new unit tests are checking the correct values and tests pass.

Submitter Checklist

Update release notes:

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[ealeksandrov]

LGTM
Great example of using Apple APIs for i18n!

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8181-09bf974 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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