feat: SplashScreen Background Color preference support

[dpogue]

Platforms affected

iOS

Motivation and Context

Closes #1254.

Description

Support a SplashScreenBackgroundColor preference that sets the background colour of the splash screen via xcassets.

If no SplashScreenBackgroundColor is provided, it will fall back to the BackgroundColor (i.e., the current behaviour). If neither are specified, it will fallback to the system background colour (also current behaviour).

Testing

• Created a new iOS project from this branch and specified both the BackgroundColor and SplashScreenBackgroundColor preferences. Ensured both variables had correct values in Xcode and confirmed splash screen used the correct colour on device.
• Created a new iOS project from this branch and specified only the BackgroundColor preference. Ensured both variables had correct values in Xcode and confirmed splash screen used the correct colour on device.

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [x] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 78.37%. Comparing base (1974dbb) to head (a9da7e2).

Files	Patch %	Lines
lib/prepare.js	52.94%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1411 +/- ## ========================================== - Coverage 78.50% 78.37% -0.13% ========================================== Files 16 16 Lines 1814 1827 +13 ========================================== + Hits 1424 1432 +8 - Misses 390 395 +5 ```

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[dpogue]

Docs PR: https://github.com/apache/cordova-docs/pull/1348