feat: group discounts

[field123]

Describe your changes

• grouping discounts from cart and item level discounts into single accessor functions
• removing rxjs and adding simple native observable pattern impl

Issue ticket number and link

n/a

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• I've added a Changeset for my changes - Click here to learn what changesets are, and how to add one

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[changeset-bot]

🦋 Changeset detected

Latest commit: dab6c2f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@elasticpath/d2c-schematics	Minor
@elasticpath/react-shopper-hooks	Minor
composable-cli	Patch

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