Added Linear Search

[codingdefined]

Part 1 (Linear Search) of Issue https://github.com/bashbers/csharp-algorithms/issues/1

• [x] Have you followed the guidelines in our Readme.md document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Have you created tests for your algorithm?

If you can check all of the above, congrats! :sparkles:

Description:

We are using the Existing Node class of csharp-algorithms. In Linear Search we are starting from the left most value of the list and if the value matches returning true otherwise false.

[bashbers]

Excellent work. I'll merge this with master. Thanks for your contribution!