Button - icons should now be passed to icon prop instead of as children

[thatblindgeye]

Follow up to breaking change PR patternfly/patternfly-react#10663

API of Button is being tweaked, icons should always be passed to icon prop. Codemod should move icons when the plain variant is applied, as that is the safest assumption that only an icon is being passed in.

Required actions:

1. Build codemod
2. Build test
3. Update readme with description & example