Message Tagger

1. Complete the tagMessages function. It should take a slice of sms messages, and a function (that takes an sms as input and returns a slice of strings) as inputs. And it should return a slice of sms messages.
   • It should loop through each message and set the tags to the result of the passed in function.
   • Be sure to modify the messages of the original slice using bracket notation messages[i].

2. Complete the tagger function. It should take an sms message and return a slice of strings.
   • Return an initialized slice, even if no tags match. No nil slices.
   • For any message that contains "urgent" (regardless of casing) in the content, the Urgent tag should be applied first.
   • For any message that contains "sale" (regardless of casing), the Promo tag should be applied second.

Example usage:

messages := []sms{
	{id: "001", content: "Urgent! Last chance to see!"},
	{id: "002", content: "Big sale on all items!"},
	// Additional messages...
}
taggedMessages := tagMessages(messages, tagger)
// `taggedMessages` will now have tags based on the content.
// 001 = [Urgent]
// 002 = [Promo]

The go strings package, specifically the Contains and ToLower functions, can be very helpful here!