Add a tooltip for elapsed time since the delivery

It adds the file util/time_ago.lua in order to re-use the function further

util/time_ago.lua

@@ -0,0 +1,29 @@
+-- translation
+local S = minetest.get_translator("mail")
+
+function mail.time_ago(t)
+    local elapsed = os.time() - t
+    local str = ""
+
+    local time_units = {
+    { S("years"), 31536000 },
+    { S("months"), 2592000 },
+    { S("weeks"), 604800 },
+    { S("days"), 86400 },
+    { S("hours"), 3600 },
+    { S("minuts"), 60 },
+    { S("seconds"), 1 },
+    }
+
+    for _, u in ipairs(time_units) do
+        local n = math.modf(elapsed/u[2])
+        if n > 0 then
+            str = str .. " " .. n .. " " .. u[1]
+            elapsed = elapsed - n * u[2]
+        end
+    end
+
+    str = string.sub(str, 2, -1)
+
+    return S("@1 ago", str)
+end