Add WorkItem description widget to GraphQL API

app/assets/javascripts/graphql_shared/possible_types.json

app/graphql/types/work_item_type.rb

app/graphql/types/work_items/widget_interface.rb

app/graphql/types/work_items/widget_type_enum.rb

app/graphql/types/work_items/widgets/description_type.rb

app/models/work_item.rb

app/models/work_items/type.rb

app/models/work_items/widgets/base.rb

app/models/work_items/widgets/description.rb

doc/api/graphql/reference/index.md

lib/gitlab/graphql/markdown_field.rb

spec/graphql/types/work_item_type_spec.rb

spec/graphql/types/work_items/widget_interface_spec.rb

spec/graphql/types/work_items/widget_type_enum_spec.rb

...graphql/types/work_items/widgets/description_type_spec.rb

spec/lib/gitlab/graphql/markdown_field_spec.rb

spec/models/work_item_spec.rb

spec/models/work_items/type_spec.rb

spec/models/work_items/widgets/base_spec.rb

spec/models/work_items/widgets/description_spec.rb