Add an optional offset to File.readBytes().

test/io/file/read_bytes_from.wren

@@ -0,0 +1,20 @@
+import "io" for File
+
+var file = File.open("test/io/file/file.txt")
+
+// Zero starts at the beginning.
+System.print(file.readBytes(3, 0)) // expect: thi
+
+// Starts at the offset.
+System.print(file.readBytes(8, 3)) // expect: s is a t
+
+// Allows zero.
+System.print(file.readBytes(0, 4).bytes.count) // expect: 0
+
+// A longer number length reads until the end.
+System.print(file.readBytes(100, 2)) // expect: is is a text file
+
+// An offset past the end returns an empty string.
+System.print(file.readBytes(100, 30).bytes.count) // expect: 0
+
+file.close()