fix: don't overwrite default options unless given new value

[benmccann]

Description

If you set a single option in commonjsOptions or dynamicImportVarsOptions then the defaults for those options (e.g. commonjsOptions: { include: [/node_modules/], extensions: ['.js', '.cjs']} will get overridden on line 259. I'm guessing that's not intended or else lines 240 & 245 could simply be removed

Additional context

I was investigating an issue that turned out to be an issue in @rollup/plugin-commonjs and was reading the code here when I stumbled across this

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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