Refactor Accordion to extend from a GOVUKFrontendConfigurableComponent class

[romaricpascal]

What

After regrouping the helpers related to configuration in their own module, add a new GOVUKFrontendConfigurableComponent class to the module and use it in the Accordion to replace how the accordion gathers the different configuration to merge and merges them.

Why

This will factor some shared code across components and allow us to provide the helper as a public API to our users.

Providing this as a function will allow its name to be mangled by minifiers, and, as discovered during the spikes, make it easier to type.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Helpers related to configuration (normaliseDataset, mergeConfigurations and validateConfig) have been moved to a common/configuration.mjs file
• A new GOVUKFrontendConfigurableComponent helper function has been added to the file, with its own test suite
• The Accordion component uses this new function to compute its configuration

[romaricpascal]

If it's not too late, I think we can add extractConfigByNamespace to the functions to move inside common/configuration.mjs.