[skeleton] Make cart banner optimistic

[frontsideair]

WHY are these changes introduced?

While the cart sidebar is optimistic, the banner isn't, which is inconsistent

WHAT is this pull request doing?

It makes the cart banner count optimistic

HOW to test your changes?

1. Add something to cart
2. Quickly close the cart sidebar
3. Cart banner should show 1
4. Reopen the cart sidebar
5. Delete the line item
6. Cart banner should show 0

Post-merge steps

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[scottdixon]

Thanks for another contribution @frontsideair! Subtle but important piece of the UX 🙌 Left a few comments.

[frontsideair]

Thanks for the review! I made the changes, except my note about totalQuantity.

[frontsideair]

@scottdixon can I ping you on this, if you have the bandwidth? Also for #2503 and #2506

[scottdixon]

Thanks again @frontsideair!

Left a couple of comments and ccing @wizardlyhel for a second set of eyes 🙏

[frontsideair]

@scottdixon thanks for the review, sent a fix just now.