Added length instance Variable in singly_linked_list to keep track of length while addition/deletion

[symabbas]

Describe your change:

Fixes: #5315

• 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 have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[cclauss]

Fixes : #5315 is a terrible title for a pull request because if forces the reviewer to manually look up PR #5315. Instead the title should tell what problem the PR fixes and the body of the commit message should contain Fixes : #5315. This does three things...

1. Tells the reviewer just by (reading the PR title) what this PR does.
2. Gives the reviewer a link (Singly_Linked_List: extra O(n) time complexity while getting length of the list #5315) to click on to see the issue that this PR fixes.
3. Allows GitHub to autoclose the issue when the PR is landed.
   • https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue

[cclauss]

This is an extra piece of state that must be calculated and that calculation can be a source of bugs. It is much more pythonic to call the len() function than it is to maintain an instance variable.