[Kotlin] Implement new Concept Exercise: functions-basic

This issue describes how to implement the `functions-basic` concept exercise for the Kotlin track.

## Goal

The goal of this exercise is to show the student basic information about Kotlin functions.

## Things to teach
- How to define a function.
- How to define default values for function arguments (show official Kotlin reference for this).
- How to return a value from a function.
- (Optionally) Single-expression functions.

## Things not to teach
- Unit functions.
- Higher-order functions.
- Functions as a class members.
- Extension functions etc.

## Concepts
- `function-declaration`.
- `function-default-arguments`.

## Prerequisites

✔ None.

## Resources to refer to
- [(Kotlin Docs) Function declarations](https://kotlinlang.org/docs/reference/functions.html#function-declarations).
- [(Kotlin Docs) Default arguments](https://kotlinlang.org/docs/reference/functions.html#default-arguments).

## Representer
- TODO

## Analyzer
- TODO

## Implementing
- TODO

## Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.