Add activity selection new algorithm

[Hardvan]

Describe your change:

The Activity Selection Problem is a classic greedy algorithm problem. Given a list of activities, each with a start and end time, the goal is to select the maximum number of non-overlapping activities that can be performed. The solution involves sorting the activities by their end times and then greedily selecting the first activity that finishes the earliest, ensuring no subsequent activity overlaps with the previously selected one.

This approach ensures an optimal solution by always prioritizing the earliest finishing activities, which leaves the most room for future activities.

• [x] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• [ ] Documentation change?

Checklist:

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