feat(strings): add longest common prefix algorithm

[len1227]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• 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".

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What this adds

strings/longest_common_prefix.py — the longest common prefix (LCP) algorithm.

Given a list of strings it returns the longest string that is a prefix of every
element of the list. The list is sorted first and only the lexicographically
smallest and largest string are compared; any prefix shared by the whole list
must also be a prefix of both of those extremes, so no other pair needs to be
examined.

>>> longest_common_prefix(["flower", "flow", "flight"])
'fl'
>>> longest_common_prefix(["dog", "racecar", "car"])
''

The doctests cover empty input, single-element input, strings that diverge at
the first character, and a ValueError raised for a non-string element.

Reference: https://en.wikipedia.org/wiki/Longest_common_prefix