Add prevfile module and test file

spec/prevfile_spec.lua

@@ -0,0 +1,9 @@
+describe("prevfile", function()
+	local prevfile = require "prevfile"
+
+	it("returns nil when target directory is empty", function()
+		previous = prevfile.prevfile("dummy/")
+
+		assert.are.equals(nil, previous)
+	end)
+end)

src/prevfile.lua

@@ -0,0 +1,7 @@
+local prevfile = {}
+
+function prevfile.prevfile(target_directory)
+	return nil
+end
+
+return prevfile