Avoid creating multiple instances of DefaultTheme.

[mc-public]

The UIColor(named:in:compatibleWith:) initializer method loads colors from the module's resource file on a global queue. Classes like LineController in the Runestone project create multiple instances of DefaultTheme during initialization, causing a large number of UIColor construction tasks to be allocated on the global queue. This can lead to noticeable lag, especially for larger texts, and prevent the editor from properly releasing memory when exiting.

The changes made in this pull request are as follows:

extension DefaultTheme {
    public static let share = DefaultTheme()
}

Then replace most occurrences of DefaultTheme() in the framework with DefaultTheme.share.