feat: support given firstweekday

[usharerose]

Support parameter firstweekday which define the first day of a week

0 is Monday, and 6 is Sunday.

It can represent several week number system:

• firstweekday=0: ISO week (from Monday through Sunday)
• firstweekday=6: US system (from Sunday through Saturday)

However, the definition of one year / month's first week would still follow this rule referred to ISO 8601:

the fourth day of a week determine this week's affiliation since it presents the majority (4 or more) of a week

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
__init__.py	3	0	100%
api.py	7	0	100%
app.py	57	8	85%	46, 52, 68–69, 95–96, 121–122
date_utils
__init__.py	29	8	72%	23, 36–39, 44–46
common.py	185	9	95%	470, 481, 520, 532, 564, 589, 599, 610, 620
date_granularity.py	118	7	94%	25, 41, 51, 65, 74, 89, 98
date_range.py	40	0	100%
offset_granularity.py	25	0	100%
TOTAL	464	32	93%

Tests	Skipped	Failures	Errors	Time
144	0 💤	0 ❌	0 🔥	0.474s ⏱️