Describe what this PR does / why we need it

• Updates type annotations to make it easier to work with the codebase
• Simplifies the logic of some small functions
• uses vim.json.encode/decode instead of vim.fn.json_encode/decode
• Adds better error messages to repo/number parsing to make it easier to catch regressions like Error with Oct Autocommands #841

Does this pull request fix one issue?

NONE

Describe how you did it

• I checked the code to manually infer better type annotations for functions signatures.

Describe how to verify it

The tests should still pass after the small refactors

Special notes for reviews

Checklist

• Passing tests and linting standards
• Documentation updates in README.md and doc/octo.txt