chore: add capacity, retryDelay, and events helpers to @launchdarkly/eventsource

[joker23]

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• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26360 bytes
Compressed size limit: 29000
Uncompressed size: 129188 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 32512 bytes
Compressed size limit: 34000
Uncompressed size: 116455 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 41678 bytes
Compressed size limit: 44000
Uncompressed size: 225516 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 180265 bytes
Compressed size limit: 200000
Uncompressed size: 833635 bytes

[joker23]

Closing this stack -- pivoting to a multi-platform package architecture (see SDK-2924). Branches kept for reference; the Node implementation will be reused under a new /node subpath.