Add scrollbar to multiline TextInput

[chiuam]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

Addresses #1364. This change adds scrollbar to multiline TextInput to indicate the text view is scrollable when possible.

Changelog

[Fixed] [macOS] - Add scrollbar to multiline TextInput

Test Plan

Before: No scrollbar when the text view is scrollable.

Before.mov

After: Scrollbar is shown after the text view becomes scrollable. (Note: Scrollbar remains visible if the text view is no longer scrollable.)

After.mov

[Saadnajmi]

@christophpurrer Do you expect any issues to your side with this change? If so, like scrollEnabled, we can make showsVerticalScrollIndicator a configurable prop

[christophpurrer]

I think @shwanton is the best person to answer that question.
In doubt a configurable property to opt-in sounds good to me ;-)

[chiuam]

Opened #1368 as a followup