Implement Subset feature

[ananyatimalsina]

This pull request introduces a new feature called Subsets, which are subsets of "Vocs" (vocabularies) that share common "Words" (entries). This change allows for the grouping of related Words within a specific Voc. A new database table has been created to store Subsets, each consisting of a range (from - to) and a vocId, which ties it to a specific Voc.

Changes:

database.dart:
    Added the implementation of the Subset model and the corresponding logic to store and retrieve subsets from the database.

dismissible_card.dart:
    Added to enhance the reusability of the Dismissible widget to be used with the subset and word editors respectively.

subset_card.dart:
    Added a visual representation for subsets, mirroring the structure of word_card.dart.

subset_editor_card.dart:
    Introduced SubsetEditorCard, similar to word_editor_card.dart, but customized for editing subsets.

voc_details_page.dart:
    Integrated the main logic for handling subsets within the existing Voc detail view, using the sublist() method for managing subsets.

voc_editor_page.dart:
    Added functionality to allow users to create and edit subsets in the voc editor.

This change improves the organization and management of vocabularies by allowing users to work with related subsets of words. It provides a more granular control over vocabularies, thus making it easier to manage larger datasets and improving overall usability.

[judemont]

Wow🤩 good job!
Thank you so much for your contributions. It looks great and I will review this pull request this weekend when I have time.

[judemont]

There are a few things I'd like to improve later, but for the moment everything seems fine.
Thanks again for your help.

[judemont]