feat(datetime): add showAdjacentDays to display days from the previous and next months (#30262)

Issue number: Internal


## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->
Adds a new property to datetime, showAdjacentDays, that when true will
show the last days of the previous month and the first days of the next
month. This will just occupy empty "cells" at the beginning of the month
"table" and add rows to the table until a maximum of 6 rows are
displayed.

## Changes
- add styles for adjacent day button
- add `showAdjacentDays` property to datetime component
- change month generation to respect new property
- add visual tests to new feature

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information


[Preview](https://ionic-framework-git-rou-11118v2-ionic1.vercel.app/src/components/datetime/test/show-adjacent-days)

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Co-authored-by: Brandy Smith <brandyscarney@users.noreply.github.com>
Co-authored-by: Brandy Smith <6577830+brandyscarney@users.noreply.github.com>
Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>