Create 05_CaseSwitcher.cs

05. You are given a text. Write a program that changes the text in all regions surrounded by the tags <upcase> and </upcase> to uppercase. The tags cannot be nested.
Example:
We are living in a <upcase>yellow submarine</upcase>. We don't have <upcase>anything</upcase> else. 

The expected result:
We are living in a YELLOW SUBMARINE. We don't have ANYTHING else.