Refactor `elasticsearch` module for modular and extensible filter handling

[cewald]

In order to the PR https://github.com/DivanteLtd/storefront-query-builder/pull/4#issuecomment-609625727 and #3, we decided to give a more extensible approach in adding features to the storefront-query-builder by doing some refactoring.

I refactored the src/elasticsearch/index.ts and put the logic into a class to make it more read- and extendable. I also added the following features:

• Add applySort method to SearchQuery to add more complex, custom sorts from SearchQuery
• Add customFilters option to buildQueryBodyFromSearchQuery to be able to extend SearchQuery by custom filters
• Add or, nor and nin filters to baseFilters
• Refactor buildQueryBodyFromSearchQuery into more modular class – The goal was to have more readable code and be able to extend the base filter functions in a more logical way
• Added some eslint and typescript improvements for better coding experience

There are some pitfalls and thoughts about backwards compatibility to do yet:

• The unit tests of the VSF will fail with this despite the new _applySort property in SearchQuery – this is really easy to fix, but would appear in each test running against old code
• The API would need an extension to somehow support the new customFilters catalog-endpoint-wise without a need to overwrite this endpoint
• The api-search-query adapter in the Vue Storefront will need some small updates for the new sorting options

So it would be a good idea to manage this changes under a new version number to support older systems by the current version and just use this version from the current VSF/API develop branches on as a new version.

This is currently in progress and not much tested – but please give me some feedback if I don't head in a completely wrong direction.

[cewald]

I have tested it a bit now and it seems to work fine. I needed to make some changes at the API and VSF to make it work.

Changes for vue-storefront - https://github.com/DivanteLtd/vue-storefront/pull/4234:

• Update the Unit-Tests to accept the SearchQuery object
• Update api-search-query search-adapter to support new storefront-query-builder sorting
• Connect new branch or npm version of storefront-query-builder
• Update Docs – add a recipe on how to add a custom filter

Changes for vue-storefront-api - https://github.com/DivanteLtd/vue-storefront-api/pull/429:

• I've added a helper that searches for specific filter classes in enabled extension folders and adds them to the catalog endpoint – this could be easily implemented on other ES queries as well this way
• Update bodybuilder package to 2.2.21 to be able to use the clone method
• Connect new branch or npm version of storefront-query-builder

Changes for storefront-api - https://github.com/DivanteLtd/storefront-api/pull/51

• I've added a helper that searches for specific filter classes in enabled extension folders and adds them to the catalog endpoint – this could be easily implemented on other ES queries as well this way
• Update bodybuilder package to 2.2.21 to be able to use the clone method
• Connect new branch or npm version of storefront-query-builder

I will open up two PR's in the repos and connect this ticket. ✔

[gibkigonzo]

Thanks, this looks great! :)