Add mergeAndConcat function

[silveltman]

similar to mergeDeep but concatting arrays with the same key, instead of overwriting.

This one is awesome, with good ts support: https://github.com/mesqueeb/merge-anything

edit: I think mergeAndConcat (or mergeConcat) would be a better name, rather then mergeDeepConcat for example. Reason being that in order to merge AND concat the value already is sure to be a combination of object and array, therefore must be deeper then one layer

[cjquines]

my proposed api here would be to reuse mergeDeep with an options param, like deno's deepMerge