Add collection support for belongs_to fields

[kaka-ruto]

Fixes #156 & #176

Currently, belongs_to fields don't show all available options in the dropdown. This PR:

• Updates BelongsTo field to support a collection option for loading all available records
• Updates import map packages to use ga.jspm.io, unpkg did not have tom-select and, from the second issue, tailwind-css-stimulus-components as well

Example usage:

class ArticleResource < Madmin::Resource
  attribute :parent_article, collection: -> { Article.all }
end

Before: Static select dropdown with only the current selection visible
After: Searchable dropdown showing all available options with filtering

Testing:
1. Visit any resource edit page with a belongs_to association
2. Verify the dropdown shows all available options when collection is specified
3. The collection is searchable