**Motivation**

Whenever a user changes the system font size to its maximum allowable setting, React Native apps that allow font scaling can become unusable because the text gets too big. Experimenting with a native app like iMessage on iOS, the font size used for non-body text (e.g. header, navigational elements) is capped while the body text (e.g. text in the message bubbles) is allowed to grow.

This PR introduces a new prop on `<Text>` and `<TextInput>` called `maxContentSizeMultiplier`. This enables devs to set the maximum allowed text scale factor on a Text/TextInput. The default is 0 which means no limit.

Another PR will add this feature to Android.

**Test Plan**

I created a test app which utilizes all categories of values of `maxContentSizeMultiplier`:
  - `undefined`: inherit from parent
  - `0`: no limit
  - `1`, `1.2`: fixed limits

I tried this with `Text`, `TextInput` with `value`, and `TextInput` with children. For `Text`, I also verified that nesting works properly (if a child `Text` doesn't specify `maxContentSizeMultiplier`, it inherits it from its parent).

Lastly, we've been using a version of this in Skype for several months.