Add the stacking package

[Beliavsky]

https://cran.r-project.org/web/packages/stacking/index.html

stacking: Building Predictive Models with Stacking

Building predictive models with stacking which is a type of ensemble learning. Learners can be specified from those implemented in 'caret'. For more information of the package, see Nukui and Onogi (2023) <doi:10.1101/2023.06.06.543970>. Packages caret, parallel, snow, and packages for base and meta learners should be installed.