chore: port EventSource client to @launchdarkly/eventsource

[joker23]

Requirements

• 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

Related issues

Provide links to any issues in this repository or elsewhere relating to this pull request.

Describe the solution you've provided

Provide a clear and concise description of what you expect to happen.

Describe alternatives you've considered

Provide a clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context about the pull request here.

[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/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

[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

[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.