MudDateRangePicker: Add missing properties: PlaceholderStart/End and SeparatorIcon

[gaplin]

Description

Fixes: #5324
Added PlaceholderStart, PlaceholderEnd and SeparatorIcon properties to MudDateRangePicker.

This properties are used by MudRangeInput so even without this change it is possible to set them.
For example this code works just fine: https://try.mudblazor.com/snippet/wEQnEnkJxCIfABKW

But they were not present on the API docs and are not visible to user (I mean as autocompletion in IDE).
You could only deduce that they are present by looking into RangeInput source code).

How Has This Been Tested?

unit

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[ScarletKuro]

@henon I think we should merge this

I just found our that our usage example

MudBlazor/src/MudBlazor.Docs/Pages/Components/DatePicker/Examples/DateRangePickerUsageExample.razor

Line 6 in b13a71e

<MudDateRangePicker Label="Placeholder" @bind-DateRange="_dateRange" PlaceholderStart="Start Date" PlaceholderEnd="End Date" />

sets the PlaceholderStart/End, but IDE shows them they are missing, I don't know why the compiler doesn't give an error.

[henon]

It doesn't give an error because they are then treated as normal html arguments

[henon]

Thanks @gaplin and sorry that your PR was overlooked for almost a year