A way to disable default handle styles

[artbud]

Overview of the problem

I'm using re-resizable version [5.0.1]

Demo:
https://codesandbox.io/s/strange-raman-kwxsr

Description of the problem

The docs state the following:

#### `handleStyles?: HandleStyles;`

The `handleStyles` property is used to override the style of one or more resize handles.
Only the axis you specify will have its handle style replaced.
If you specify a value for `right` it will completely replace the styles for the `right` resize handle,
but other handle will still use the default styles.

However now passed handleStyles extend the default styles, not override.

Expected behavior

If I opt to style my handle myself, I'd like to only use the handleClasses prop and not worry about default styles. Please see the provided Codesandbox demo.

Actual behavior

This issued was addressed in #139 but the resulting code still extends the styles instead of replacing them.

Proposed solution

If replacing the styles is a breaking change, we could allow passing null in handleStyles to implicitly disable default styles.

Like so:

handleClasses={{ right: 'right-handler' }}
handleStyles={{ right: null }}

Let's discuss the solution and I'll attempt to make a PR with the changes :)