add: countSubstrings function implementation

[ajinkyac03]

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Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

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 JavaScript files are placed inside an existing directory.
• All filenames should use the UpperCamelCase (PascalCase) style. There should be no spaces in filenames.
  Example:UserProfile.js is allowed but userprofile.js,Userprofile.js,user-Profile.js,userProfile.js are not

Fixes#1090

[appgurueu]

What about overlapping substrings? I might expect countSubstrings("aaa", "aa") to be 2. Currently this is undocumented & untested behavior.

[appgurueu]

Note: Runtime is O(n²) due to the naive implementation.