perf(ext/web): add write buffering for FsFile.writable streams

[bartlomieju]

Summary

• Add opt-in write buffering to writableStreamForRid() via a new bufferSize option
• Enable 64KB buffering for FsFile.writable streams only — network sockets, stdin, QUIC, and WebTransport remain unbuffered
• Small chunks are accumulated in a JS-side buffer and flushed when full or on close(); large chunks (>= 64KB) bypass the buffer

Motivation

Closes #16667

Writing small buffers (128 bytes) via file.writable.getWriter().write() is ~100x slower than Node.js. The root cause is that every small write goes through: JS promise machinery → async op dispatch → task queue permit → spawn_blocking → syscall, with zero buffering.

With this change, 100K writes of 128 bytes drops from ~1.6s to ~0.06s (debug build).

Design

• Buffering is opt-in via writableStreamForRid(rid, autoClose, cfn, { bufferSize }) — only FsFile.writable passes it
• When bufferSize > 0, highWaterMark switches to byte-based sizing instead of count-based
• Buffer is lazily allocated on first small write
• close() flushes remaining buffer, abort() discards it

Test plan

• Existing spec tests pass
• cargo test -p deno_runtime
• Manual benchmark: 1M x 128-byte writes via file.writable.getWriter().write()
• Verify large writes (> 64KB) still work correctly
• Verify close() flushes remaining buffered data
• Verify network socket writable streams are unaffected

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