Added strings programs in data structure folder

[VeeruYadav45]

Describe your change:

i have added 4 string programs:

1. All unique
2. Anagram
3. Frequency of each words
4. Reverse words

• 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:

• 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) with a closing keyword: "Fixes #ISSUE-NUMBER".

[algorithms-keeper]

Click here to look at the relevant links ⬇️

🔗 Relevant Links

Repository:

• Contributing guidelines
• Project Euler solution guidelines

Python:

• Formatted string literals (f-strings)
• Type hints
• doctest
• unittest
• pytest

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[algorithms-keeper]

As there is no test file in this pull request nor any test function or class in the file data_structures/strings/allunique.py, please provide doctest for the function has_unique_chars

Please provide return type hint for the function: has_unique_chars. If the function does not return a value, please provide the type hint as: def function() -> None:

Please provide type hint for the parameter: s

Please provide descriptive name for the parameter: s

[algorithms-keeper]

As there is no test file in this pull request nor any test function or class in the file data_structures/strings/anagram.py, please provide doctest for the function are_anagrams

Please provide return type hint for the function: are_anagrams. If the function does not return a value, please provide the type hint as: def function() -> None:

Please provide type hint for the parameter: s1

Please provide type hint for the parameter: s2

[algorithms-keeper]

As there is no test file in this pull request nor any test function or class in the file data_structures/strings/frequencyofeachchar.py, please provide doctest for the function char_frequency

Please provide return type hint for the function: char_frequency. If the function does not return a value, please provide the type hint as: def function() -> None:

Please provide type hint for the parameter: s

Please provide descriptive name for the parameter: s

[algorithms-keeper]

As there is no test file in this pull request nor any test function or class in the file data_structures/strings/reverseword.py, please provide doctest for the function reverse_words

Please provide return type hint for the function: reverse_words. If the function does not return a value, please provide the type hint as: def function() -> None:

Please provide type hint for the parameter: sentence

[cclauss]

Closing require_descriptive_names PRs to prepare for Hacktoberfest