Add an optional offset to File.readBytes().

test/io/file/read_bytes.wren

@@ -10,4 +10,10 @@ System.print(file.readBytes(7)) // expect: this is
 // Allows zero.
 System.print(file.readBytes(0).bytes.count) // expect: 0
 
+// A longer number reads the whole file.
+System.print(file.readBytes(100)) // expect: this is a text file
+
+// Reading past the end truncates the buffer.
+System.print(file.readBytes(100).bytes.count) // expect: 19
+
 file.close()