fix(fs): truncate the file when writeFile writes a stream (#3629)

In plugins/fs/guest-js/index.ts writeFile, the stream path opened the
file with {write, create} but no truncate, unlike the buffer path which
truncates unless `append` is set (commands.rs write_file_inner), so
shorter content left stale trailing bytes.

The stream path now passes `truncate: !options?.append`. api-iife.js
regenerated. E2E spec covers overwrite and append with a stream.

.changes/fix-fs-write-stream-truncate.md

@@ -0,0 +1,6 @@
+---
+fs: patch
+fs-js: patch
+---
+
+Fixed `writeFile` with a `ReadableStream` leaving the end of the previous contents in place when the new data is shorter. The file is now truncated first unless `append` is set, like when writing a buffer.