Fix shallow copying by repeatAppend()

[jacobtylerwalls]

Fixes #176

[mscuthbert]

common.merge is designed to work on {} objects not [] array. Probably needs a line before the l. 143 if to ask if this[key] instanceof Array and then do a deep array copy.

[mscuthbert]

I'm guessing this is not enough. Probably need to do something more like:

Split the section beginning:
for (const key in this) {
into its own method (common.cloneHelper?) (i.e. a function that doesn't use "this" but obj or something like that). And then when encountering another dictionary/object, calling cloneHelper on that.

[mscuthbert]

Now that it's 2024, I wonder if we should just be replacing 90% of these calls with structuredClone() and saying that anything beyond the top-level keys needs to be explicitly listed as having its own clone function or needs to be clonable with structuredClone...