feat: add SSE streaming helper support

Add Server-Sent Events (SSE) and streaming response support with three
streaming functions:

- stream(): Basic binary streaming with chunked transfer encoding
- stream_text(): Text streaming with proper Content-Type headers
- stream_sse(): SSE event streaming with full event field support

Features:
- SSEEvent struct with data, event, id, retry fields
- StreamContext for write, writeln, write_sse, sleep, pipe operations
- Multi-line data handling in SSE events
- Custom error handler support
- Client disconnect detection via on_abort callback
- Integrated with both Picoev and uSockets backends

New files:
- streaming.v: Core streaming implementation
- examples/sse_demo.v: Comprehensive SSE demo with test page
- tests/test_stream_*.v: 62 property-based tests

Updated:
- picoev_server.v: PicoevStreamWriter integration
- usockets_server.v: UsocketsStreamWriter integration
- README.md: SSE streaming documentation

v.mod

@@ -1,7 +1,7 @@
 Module {
 	name: 'hono'
 	description: 'A high-performance V language web framework inspired by Hono.js, featuring hybrid routing, LRU cache, middleware support, and more.'
-	version: '0.6.0'
+	version: '0.7.0'
 	license: 'MIT'
 	dependencies: []
 }