colorPropsReplacedColors rule - update variable declaration instead of prop value

[thatblindgeye]

Followup based on #691 (comment). We should update the rule so that the fixer updates any var declarations instead of always the prop value. We could use this as an excuse to create some helpers as well for general var updating.

This could involve creating a new helper and using it any other applicable rules.

Required actions:

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