collapsedSize and buttonPositionOffset adjustments

[gweis]

Hi, ... very nice package.

I think I have found 3 small issues though.

1. react-collapse-pane/src/components/SplitPane/hooks/callbacks/useUncollapseSize.ts

   Line 24 in 8fe33bd

   const offset = isReversed ? -(size - 50) : size - 50;

   
   this line adjust the offset by 50 pixels (hardcoded), but I think it should use collapsedSize for both

2. https://github.com/b-zurg/react-collapse-pane/blob/master/src/components/Resizer/index.tsx#L115
   when creating the flex style for prevButtonFlex and postButtonFlex the flex-basis needs a unit, otherwise browser like chrome will ignore the applied style.

3. The examples in https://github.com/b-zurg/react-collapse-pane/blob/master/docs/README.md pass collapseOptions as property to SplitPane, but I think the prop should be named collapse.

cheers