Add Merge Sort in F#

[NurMohamed-system]

To request a new code snippet, please fill out the following:

Project name: Merge Sort in F#
Project link: https://sampleprograms.io/projects/my-project-name
Language: f-sharp

If this code snippet will also be the first in a new language, please fill out the following:

Official Language Style Guide:
Official Language Website:
Official Language Docker Image: https://hub.docker.com/r/repository/image

The project link must be a link to a project on the Sample Programs Website project list.
If you would like to add a new project to the Sample Programs Website,
please first make a pull request to the Sample Programs Website.

[Kaamkiya]

Have fun :)