fix: append on_new_config instead of replacing it

[kylo252]

• Append to on_new_config instead of completely overwriting it.

This helps plugins that use this callback, such as https://github.com/tamago324/nlsp-settings.nvim

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• partially revert 03a44ec: workspace.library uses a map and not a list

Fixes #40

[captainko]

@folke please check this out.

[folke]

Thanks!