os: fix File.read() in JS backends (fix #20501) (#20633)

vlib/os/file.js.v

@@ -84,9 +84,10 @@ pub fn (f &File) read(mut buf []u8) !int {
 	}
 	mut nbytes := 0
 	#try {
-	#let buffer = $fs.readFileSync(f.val.fd.valueOf());
+	#let readBuffer = Buffer.alloc(buf.val.len.valueOf());
+	#nbytes = $fs.readSync(f.val.fd.valueOf(), readBuffer, 0, buf.val.len.valueOf(), null);
 	#
-	#for (const val of buffer.values()) { buf.arr[nbytes++] = val; }
+	#for (let i = 0; i < nbytes; i++) { buf.val.arr.arr[i] = new u8(readBuffer[i]); }
 	#}
 	#catch (e) { return error('' + e); }
 

vlib/os/open_and_read_from_file_test.js.v

@@ -0,0 +1,8 @@
+import os
+
+fn test_read_from_file() {
+	mut buf := []u8{len: 10}
+	f := os.open(@FILE)!
+	n := f.read(mut &buf)!
+	println(buf[..n])
+}