Refactor word_occurrence to use dict

[Miladkhoshdel]

Describe your change:

Refactor word_occurrence() to use a standard dict instead of defaultdict.

This change:

• Uses dict.get() to count word occurrences.
• Updates the return type annotation from dict to dict[str, int].
• Removes the unnecessary defaultdict dependency.
• Preserves the existing behavior and doctests.

• 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".

[cclauss]

Python's default_dict is in the standard library to accelerate use cases like this one. There is a function call overhead in the proposed solution that does not exist in the current solution.

Create a https://docs.python.org/3/library/timeit.html#timeit.timeit benchmark and compare the two on a big dataset.

[Miladkhoshdel]

Thanks for the feedback. I’ve kept the existing defaultdict implementation unchanged and moved the type-hint and typo fixes into a separate, focused PR: #15194.