Increase Compression Streams output buffer size

Since zlib uses 256KiB of memory per compression stream with default settings, a
size of 16KiB will not increase memory usage by a large proportion. This should
make processing data slightly faster. Ideally we'd want this to be larger than
deflate's 32KiB window size which would allow zlib to avoid some extra copying
during decompression, but that can be done at a later time alongside other
compression optimizations.

src/workerd/api/streams/compression.c++

@@ -129,7 +129,7 @@ private:
 
   Mode mode;
   z_stream ctx = {};
-  kj::byte buffer[4096];
+  kj::byte buffer[16384];
 
   // For the eponymous compatibility flag
   ContextFlags strictCompression;