Support for UIContentSizeCategoryAdjusting

[ptrkstr]

Is your feature request related to a problem? Please describe.
Currently apps which utilise UILabel, UITextView and UITextField are able to support dynamic type via opt in of adjustsFontForContentSizeCategory. Once a developer enables this property, users are able to have an app experience that caters to their font size preferences for example:

• Larger font for users with difficulty seeing.
• Smaller font for users which want to display more content at once.

Describe the solution you'd like

• TextView conforms to UIContentSizeCategoryAdjusting.
• adjustsFontForContentSizeCategory can be set per TextView instance.
• Internally this updates the fonts for both line number and text editor views.

Describe alternatives you've considered
Without modifying the original source code, a workaround currently is to set a new theme with updated fonts every time a user changes font size. This approach however doesn't seem as performant as only updating internal font properties directly.

Additional context

• UIContentSizeCategoryAdjusting
• Scaling Fonts Automatically
• Text Size and Weight

[simonbs]

This seems like a great idea, and I'd like Runestone's TextView to behave as closely to UITextView as possible, so I think we should add this.

Sorry for the late answer!