feat: add prepend slot to date/time control renderers

[yaffol]

Adds optional prepend slot to DateControlRenderer, DateTimeControlRenderer, and TimeControlRenderer to allow custom content before the input field.

Benefits:

• Enables custom icons/buttons before input field
• Follows existing Vuetify slot pattern (prepend-inner already exists)
• Backward compatible (slot is optional)
• Consistent across all date/time renderers

Use Case: Allows adding info/help icons or other custom content in the prepend area of v-text-field.

Testing: All existing tests pass. The conditional application with v-if="$slots.prepend" prevents and empty <div class="v-input__prepend"> being injected to the rendered html.

Extension: If this pattern is acceptable, I could extend it across all the renderers, to allow access to the prepend slot for customisation.

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[coveralls]

coverage: 82.798%. remained the same
when pulling 722ef61 on yaffol:feature/add-prepend-slot
into 0af54c5 on eclipsesource:master.

[lucas-koehler]

Hi @yaffol , thanks for the contribution ❤️
I have one question: What is the exact use case of this? I.e. as I understand it, using the additional slot allows custom renderers using these controls to fill the prepend slot but the renderers themselves don't use them or add anything there.

Is this correct?