the_weekday_date() should use is_new_day()

[SergeyBiryukov]

Background: #33750, #46627.

Since [34474], the_date() uses is_new_day() instead of directly comparing $currentday and $previousday globals.

the_weekday_date() should do the same for consistency.

[SergeyBiryukov]

In 45377:

Docs: Improve documentation for the_date() and the_weekday_date().

Correct type for $currentday and $previousday globals.

See #47354, #47110.

[SergeyBiryukov]

In 45378:

Date/Time: Bring some consistency to the_date() and the_weekday_date():

• Make the_date() always apply the the filter and return a value.
• Use is_new_day() in the_weekday_date().
• Add a unit test for the_weekday_date().

Fixes #47354.

[SergeyBiryukov]

In 45379:

Date/Time: Return early from the_weekday() and the_weekday_date() if we're not in the loop.

Restore $previousweekday global usage in the_weekday_date(), so it could still be used simultaneously with the_date().

Partially reverts [45378].
See #47354.