Added new algorithm to generate numbers in lexicographical order

[1227haran]

Describe your change:

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

[1227haran]

All the tests are passing, please approve the PR

[1227haran]

Instead of returning a string, can we have the function be a generator that returns individual numbers? That way, a user can iterate over the numbers and do with the output as they please.

For instance, if a user wanted a list of the numbers in lexicographical order, they could easily do

list(lexical_order(20))

and if they wanted a string, they could easily do

" ".join(lexical_order(20))

That's a great suggestion, @tianyizheng02 ! I've updated the code, and now it's more flexible. Let me know if there are any other changes.