The font of ui5-textarea's placeholder should be "slanted" text (italics)

[GongRichard]

Describe the bug
The font of ui5-textarea's placeholder should be "slanted" text (italics).

To reproduce
Steps to reproduce the behavior:

1. Go to https://sap.github.io/ui5-webcomponents/playground/components/TextArea/
2. The placeholder text is not in italics.

Expected behavior
The placeholder text should be "slanted" text (italics).
Check https://sapui5.hana.ondemand.com/#/entity/sap.m.TextArea/sample/sap.m.sample.TextAreaGrowing

Context

• UI5 Web Components version
• OS/Platform: {...}
• Browser (if relevant): {...}
• Other information: {...}