Added Llinear Search, used Error-Set and 8 test cases.

[therexdy]

Added file search/linearSearch.zig:

• search function takes a slice and the key value as arguments
• returns an error union !usize
• returns usize index if key is found
• if not found returns KeyNotFound error from the Error-Set searchErrors

Included test cases to test off-by-one, empty array, single element, duplicate key values in array etc.

zig version 0.15.2

[kassane]

@therexdy , nice contribution.