Added CupertinoDatepicker monthYear mode (flutter#93508)

[lindsaypj]

This PR adds a month and year mode to the CupertinoDatePicker. The monthYear mode is the date mode without the day of the month.

This feature was added at the request of:
[Proposal] CupertinoDatePicker with month and year. #93508

One thing that I was unsure of was the use of the DatePickerDateOrder to determine the monthYear order. It could be considered a workaround since the DatePickerDateOrder is intended to order day, month, and year. This means that a developer could use the DatePickerDateOrder.dmy (day, month, year) or DatePickerDateOrder.mdy (month, day, year) to get the same result.

At first I intended to add a DatePickerMonthYearOrder enum to the localizations, in addition to a new parameter for the CupertinoDatePicker for monthYearOrder, but I ended up reverting these changes (1c61f10) because I had not considered the effects of adding values to the localizations.

I decided it may be better to not add an additional parameter (monthYearOrder) that would go mostly unused. I am very open to feedback or ideas on this matter.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[MitchellGoodwin]

I agree with not adding a new property to the constructor or enum to dateOrder just to reorder the picker when in monthYear mode. It adds bloat to the API when the functionality can be handled by what currently exists. In exchange, could you add something explaining how it would work in the documentation for dateOrder? I think that's the most natural approach.

Besides that, this LGTM to me. Thank you for taking the time to do this.

[lindsaypj]

Thank you Mitchell! I updated the dateOrder documentation, noting that there are multiple DatePickerDateOrder options that result in the same column order.

[MitchellGoodwin]

LGTM. Thank you for taking the time to do this!

[justinmc]

LGTM 👍. Thanks for the solid PR and all of the tests!

The Google testing failure seems like it might be unrelated... Could you push a merge commit @lindsaypj?

[MitchellGoodwin]

@lindsaypj it looks like the google testing is stuck. Would you mind rebasing in order to restart it? That's normally the most reliable method.

[lindsaypj]

I'm not sure how, but I added commits from master. I created another branch from commit 4f8a45d.

date_picker_month_year_mode

Should I just create another PR, or is there a way to easily remove all the additional commits I created in my failed rebase attempt?

Edit: Actually I managed to revert back to before the rebase. If the google test still doesn't resolve, I will try the rebase again. I was shown the correct way to do it.