fix(mergeConfig): don't recreate server.hmr.server instance

[Vija02]

Description

Currently, the mergeConfig function recursively goes through objects and overrides them. This works for the most part, except for the server.hmr.server property which is a Server class/object.

The problem is that the mergeConfig function treats it like a normal JS object and effectively clones it. This turns it into a standard object which doesn't work. And it also breaks reference to the object.

We either need to modify the isObject function to check for classes, or for now, I've added an if statement to handle the special case, following the other precedence.

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[bluwy]

I also think it's fine to keep it simple and handle this specific case for now. We can revisit checking classes next time.