[feat] add focus style props for non DOM renderers

[JohnPaulHarold]

This PR:

• adds focusedStyle, focusedLeafStyle, disabledStyle and activeStyle props. These props merge a given style object to the existing style object prop for the inner component.
• adds the optional chaining babel plugin(https://babeljs.io/docs/en/babel-plugin-proposal-optional-chaining). I needed this while importing the lib locally, via file:../etc. Running npm run build and then importing your forked lib via file:../path/to/lrud wasn't recognising the optional chaining here, https://github.com/jamesplease/lrud/blob/master/src/utils/create-node.ts#L77 This is important as with this fix I can resume creating replicas of the /examples folder, using revas instead
• tested on the revas renderer, which uses similar styling idioms to React Native

Example

const styles = {
  view: {
    borderWidth: 1,
    borderColor: 'transparent',
    borderStyle: 'solid'
  },
  focused: {
    borderColor: 'magenta'
  }
}

<FocusNode elementType={View} style={styles.view} focusedStyle={styles.focused}/>

Notes

My motivation for writing this was to use the lrud lib with a non ReactDom renderer, in this case revas.

Issues

• I had difficulties writing a test for the <FocusNode/> component, as to do so would require react and react-dom. Adding those, even if only for a test, seems to include them in the bundle deps and this then causes conflicts with react projects wanting to use lrud. I think the issue is related to the output of the build including *.test.ts{x} files as well, but am not sure, and also could not work out how to make babel not include such files when building.
• with the above in mind, I buffed up the coverage for the warning() util, which gets a green tick on the pipeline, but is perhaps against the intent of the coverage. Certainly feels like it's gaming it a bit.
• I've copied some of the examples, modifying them for the alternate renderer, and so far have basic, grid, trap working much like the react-dom equivalents. I will push them to my github repo soon.

[JohnPaulHarold]

@jamesplease after a bit of a false start, this PR is ready enough for review now.

[JohnPaulHarold]

closing as, in its current form, it only applies a style to the top of the component, not much use if you want to transform a node inside of the passed element.

As it is, you can still tokenise the classname string and ascertain the focus state that way, so will work that way for now.