wrong mapping for 'textFontSize' property in button component

## 🐛 Bug Report

When using `Button` component, mapping provide `textFontSize` for setting up `fontSize` for text inside button. Yet, looks like mappings doesn't work for this particular property. When trying to set up different `textFontSize` - it's always applies default value, which if `16`.

It should work, since `textFontSize` gets from eva mapping [here](https://github.com/akveo/react-native-ui-kitten/blob/04481cec49a78f859c4cde65a45b157770ea1439/src/components/ui/button/button.component.tsx#L173). But that value always returns `16`.

### Might related to eva package instead!

## To Reproduce

Steps to reproduce the behavior:
Create button component and try to modify mapping by changing `textFontSize`. Get button element ref and check what value eva returns

## Expected behavior

Mapping should get correct value.

## UI Kitten and Eva version

| Package | Version |
| ----------- | ----------- |
| @eva-design/eva | 2.1.0 |
| @ui-kitten/components | 5.1.0 |

## Environment information

```bash
System:
OS: macOS 11.1
CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
Binaries:
Node: 14.15.4 - ~/.nvm/versions/node/v14.15.4/bin/node
Yarn: 1.22.10 - /usr/local/bin/yarn
npm: 7.14.0 - ~/.nvm/versions/node/v14.15.4/bin/npm
Watchman: 4.9.0 - /usr/local/bin/watchman
SDKs:
iOS SDK:
Platforms: iOS 14.5, DriverKit 20.4, macOS 11.3, tvOS 14.5, watchOS 7.4
Android SDK:
API Levels: 28, 29, 30
Build Tools: 28.0.3, 29.0.2, 30.0.3
System Images: android-23 | Google APIs Intel x86 Atom, android-29 | Google APIs Intel x86 Atom, android-30 | Google APIs Intel x86 Atom, android-30 | Google Play Intel x86 Atom
IDEs:
Android Studio: 4.2 AI-202.7660.26.42.7322048
Xcode: 12.5/12E262 - /usr/bin/xcodebuild
npmPackages:
react: 17.0.1 => 17.0.1
react-native: 0.64.0 => 0.64.0
```