Add benchmarks

[MatthewWid]

Add benchmarks for better-sse compared with other libraries for:

• [ ] Pushing events to a single client
• [ ] Pushing events to multiple clients at once

Add benchmarks for the following libraries:

• [x] better-sse
• [x] sse-channel
• [x] easy-server-sent-events
• [ ] sse
• [ ] express-sse
• [ ] sse-stream