fix(web-host): fix tee command file overwriting behavior #12

The `Descriptor#write` method was incorrectly trying to preserve existing file content
when writing, which broke the `tee` command's ability to overwrite files.

The fix simplifies the logic: instead of concatenating new data with existing file
content, the method now directly replaces the file's buffer. This matches the expected
behavior of file writes - when writing to a file, it should overwrite the content,
not append to it.

- Remove buffer concatenation logic in writeViaStream
- Implement direct buffer replacement for proper file overwriting
- Fixes tee command behavior on both new and existing files

Issue: #12

Author: topheman

packages/web-host/overrides/@bytecodealliance/preview2-shim/lib/browser/filesystem.js

@@ -110,9 +110,8 @@ class Descriptor {
     let offset = Number(_offset);
     return new OutputStream({
       write (buf) {
-        const newSource = new Uint8Array(buf.byteLength + entry.source.byteLength);
-        newSource.set(entry.source, 0);
-        newSource.set(buf, offset);
+        const newSource = new Uint8Array(buf.byteLength);
+        newSource.set(buf, 0);
         offset += buf.byteLength;
         entry.source = newSource;
         return buf.byteLength;
@@ -196,7 +195,7 @@ class Descriptor {
       statusChangeTimestamp: timeZero,
     }
   }
-  
+
   statAt(_pathFlags, path) {
     const entry = getChildEntry(this.#entry, path, { create: false, directory: false });
     let type = 'unknown', size = BigInt(0);