Remove the `data/shopify_liquid/(tags|filters|objects).yml` files

[Poitrin]

WHY are these changes introduced?

Fixes #656

WHAT is this pull request doing?

• Removes the usage of data/shopify_liquid/tags.yml, data/shopify_liquid/filters.yml, data/shopify_liquid/objects.yml in favor of the SourceIndex, which holds the most updated values for objects, filters, and tags.
• Transforms remaining .yml files to .json files, so that we only have 1 format.
• Adds some logic for existing objects, filters, tags, so that it resembles the existing .yml files content.

How to test your changes?

• Verify that code completion for tags is still working as before,
  • and that the tags mentioned in LABELS_NOT_IN_SOURCE_INDEX are now suggested too.
• Verify that code completion for objects is still working as before,
  • and that customer_address and product_variant are suggested now, but offer no further suggestions like attribute suggestions.
• Verify that code completion for filters is still working as before,
  • and that the filters mentioned in LABELS_NOT_IN_SOURCE_INDEX are now suggested too.