CLI: Fix sb add / mdx-gfm for addons with non-serializable values

[shilman]

Closes #21286

What I did

Added new helper functions appendToArray to ConfigFile, which can be used to add addons to arrays that contain complex values like require.resolve or variable references.

Note there are still other places in the code that use the old getValue/setValue method that this replaces, but they are more involved to replace so scoping them out for now.

How to test

• See attached unit tests
• Test in a sandbox
  • Edit main.js to contain addons with complex values
  • Run sb add @storybook/addon-docs and verify that it doesn't crash