Implement Parentheses Generator using Backtracking Strategy

[TruongNhanNguyen]

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] I ran bellow commands using the latest version of rust nightly.
• [x] I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• [x] I ran cargo fmt just before my last commit.
• [x] I ran cargo test just before my last commit and all tests passed.
• [x] I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• [x] I added my algorithm to DIRECTORY.md with the correct link.
• [x] I checked COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.78%. Comparing base (28be0a3) to head (eaa3919).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #713 +/- ## ======================================= Coverage 94.77% 94.78% ======================================= Files 298 299 +1 Lines 22213 22246 +33 ======================================= + Hits 21052 21085 +33 Misses 1161 1161 ```

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