perf(accounts-controller): add batch events

[ccharly]

Explanation

Add new batched events in the AccountsController to allow consumers to use multiple accounts at once when multiple accounts get updated at once in a withKeyring call (which will follow-up with a KeyringController:stateChange event).

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Additive event emission change: existing per-account events remain, and new batch events are only published when there are additions/removals, making impact to consumers opt-in and low risk.

Overview
Adds new batch messenger events AccountsController:accountsAdded and AccountsController:accountsRemoved (and exported event types) that emit arrays of accounts/IDs during KeyringController:stateChange synchronization.

Updates sync logic to publish these batch events after the existing per-account :accountAdded/:accountRemoved events (only when non-empty), and extends the test suite and changelog to cover payloads and ordering guarantees.

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[mathieuartu]

Looks good!