feat(theming): add a global --ion-placeholder-opacity variable (#28944)

Issue number: internal

---------

## What is the current behavior?
Developers are unable to globally change the placeholder opacity for
inputs, textareas, searchbars, and selects. This becomes particularly
problematic when trying to apply a high contrast theme.
 
## What is the new behavior?
- Adds the `--ion-placeholder-opacity` with a fallback of `0.6` (this
fallback is unchanged)
- Sets the `--ion-placeholder-opacity` to `0.8` for the high contrast
light & dark themes

> [!NOTE]
> The ticket says to update the standard light and dark themes to define
`--ion-placeholder-opacity: 0.6`, however, I did not do this because
this is not different than the fallback and therefore does not need to
be defined.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

Documentation pull request:
ionic-team/ionic-docs#3421

core/src/css/themes/high-contrast-dark.scss

@@ -97,6 +97,8 @@ $colors:  (
       --ion-color-#{$color-name}-shade: #{map.get($value, shade)};
       --ion-color-#{$color-name}-tint: #{map.get($value, tint)};
     }
+
+    --ion-placeholder-opacity: 0.8;
   }
 
   // Toast

core/src/css/themes/high-contrast-light.scss

@@ -88,6 +88,8 @@ $colors:  (
       --ion-color-#{$color-name}-shade: #{map.get($value, shade)};
       --ion-color-#{$color-name}-tint: #{map.get($value, tint)};
     }
+
+    --ion-placeholder-opacity: 0.8;
   }
 
   // Toast

core/src/themes/ionic.globals.scss

@@ -35,7 +35,7 @@ $screen-breakpoints: (
 // Input placeholder opacity
 // Ensures that the placeholder has the
 // correct color contrast against the background.
-$placeholder-opacity: 0.6 !default;
+$placeholder-opacity: var(--ion-placeholder-opacity, 0.6) !default;
 
 $form-control-label-margin: 16px !default;