Fix/consolidated reverse letters and reverse long words

[AnshuSharma111]

Describe your change:

Deleted reverse_long_words.py and modified code of reverse_letters.py to integrate functionality of reverse_long_words.py. Both had almost same function of reversing the letters of words in a sentence without reversing the words.

1. Deleted reverse_long_words.py
2. Implemented its functionality to reverse_letters.py because both had same function
3. Changed Directory.md to reflect changes

Fixes #8098

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

[tianyizheng02]

Suggested change

	def reverse_letters(sentence: str, length: int) -> str:
	def reverse_letters(sentence: str, length: int = 0) -> str:

Let's make the length an optional parameter so that we can still preserve the same functionality as the original reverse_letters

Please also add a test case for length = 0