feat(datetime): add part for month/year button (#27618)

Issue number: resolves #26596

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

There is no way to customize the month/year toggle button using CSS.

## What is the new behavior?
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this PR. -->

- Adds a Shadow Part to the month/year toggle button.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

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Co-authored-by: ionitron <hi@ionicframework.com>

core/api.txt

@@ -427,6 +427,7 @@ ion-datetime,css-prop,--background-rgb
 ion-datetime,css-prop,--title-color
 ion-datetime,css-prop,--wheel-fade-background-rgb
 ion-datetime,css-prop,--wheel-highlight-background
+ion-datetime,part,month-year-button
 ion-datetime,part,time-button
 ion-datetime,part,time-button active
 ion-datetime,part,wheel-item

core/src/components/datetime/datetime.md.scss

@@ -36,7 +36,7 @@
 
 :host .calendar-action-buttons ion-item,
 :host .calendar-action-buttons ion-button {
-  color: #{$text-color-step-350};
+  --color: #{$text-color-step-350};
 }
 
 // Calendar / Header / Days of Week

core/src/components/datetime/datetime.tsx

@@ -82,6 +82,9 @@ import {
  * @part time-button - The button that opens the time picker when using a grid style
  * layout with `presentation="date-time"` or `"time-date"`.
  * @part time-button active - The time picker button when the picker is open.
+ *
+ * @part month-year-button - The button that opens the month/year picker when
+ * using a grid style layout.
  */
 @Component({
   tag: 'ion-datetime',
@@ -1928,6 +1931,7 @@ export class Datetime implements ComponentInterface {
         <div class="calendar-action-buttons">
           <div class="calendar-month-year">
             <ion-item
+              part="month-year-button"
               ref={(el) => (this.monthYearToggleItemRef = el)}
               button
               aria-label="Show year picker"