preview2 shim browser filesystem writeViaStream has O(n^2) overhead due to repeatedly concatenating buffers

[TerrorJack]

See

jco/packages/preview2-shim/lib/browser/filesystem.js

Line 123 in f92eaa5

writeViaStream(_offset) {

, when this is invoked repeatedly to do a lot of small appends, there is O(n^2) slowdown due to repeatedly concatenating buffers. 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.