feat: new backtracking problem of letter combinations of a phone number

[saurabhdamle11]

Description of Change

Added a new backtracking problem of "Letter combinations of phone number", I have compiled and tested.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
Added a new backtracking problem of "Letter combinations of phone number" along with testcases.