Make day of week convention more consistent across the codebase

[sdispater]

Since the beginning, Pendulum had the convention that a week starts on Monday since it's the most common case and matches the ISO standard. However there were a few places where this convention was to enforced:

• the day_of_week property
• The d and do tokens when formatting a datetime instance.

This PR makes sure everything is consistent across the codebase. Unfortunately, this makes this change a breaking one since this changes the behavior of the elements mentioned above.

The new e and eo tokens have been introduced to get the day of week but localized base on the week_data of the locale files.

Fixes #683

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

[codspeed-hq]

CodSpeed Performance Report

Merging #731 will not alter performance

_{Comparing refactor/day-of-week-consistency (2aff736) with master (7e4f806)}

Summary

✅ 2 untouched benchmarks