useMemoryFS fd_write has O(n^2) slowdown due to repeated concatenation of buffers

[TerrorJack]

See

uwasi/src/features/fd.ts

Line 672 in 7adb986

if (requiredLength > file.node.content.byteLength) {

, fd_write always allocates a new buffer and copies over old content when appending content, which results in O(n^2) slowdown when doing a lot of small writes.

I noticed this when working on bjorn3/browser_wasi_shim#95; almost all wasi in-memory vfs implementation written in js/ts I can find has the same issue, including this one.