Concatenated merge sort, iterative merge sort and insertion merge sor…

[csana004]

…t .Fixes TheAlgorithms#8098

Describe your change:

I concatenated merge sort implementations (insertion, recursive and iterative) in the sorting algorithms folder. The code is properly documented and includes doctests.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s): "Fixes Concatenate/consolidate all algorithms with different implementations TheAlgorithms/Python#8098".