Dropped usage of Enumerable#rfind to keep Ruby 3.x support

Dropped reliance on the now-deprecated ActiveSupport::Configurable module.

• rich_text_list blocks now initialize with elements as an empty array instead of nil (#1)
• The DSLs for creating surfaces can now accept and pass a hash of attributes through to the underlying surface class itself (e.g. BlockKit.modal(title: "Hello, world!") { ... }
• BlockKit::Blocks.new can now accept either a hash of attributes or an array of blocks.

New Contributors

• @wilsonehusin made their first contribution in #1. Thanks, Wilson!

• Fixes a bug where blocks with plain_text attributes would not properly initialize with provided arguments via the DSL
• Adds many new attribute aliases (e.g. confirmation_dialog instead of confirm) and constant aliases (e.g. Datetimepicker) to allow conforming to Slack's naming or more human-readable names
• Adds support for specifying attribute aliases when initializing via the DSL. For example:

BlockKit::Blocks.new do |b|
  b.input(label: "Choose a date and time") do |input|
    input.datetime_picker(initial_datetime: Time.now) # instead of `initial_date_time:`
  end
end

• Fixes a bug where number_input with is_decimal_allowed set to false would render their min_value, max_value, and initial_value fields as integers instead of strings, failing Slack's schema requirements.

• Adds a validation and fixer to ensure that modals have a submit button when it contains input elements
• Fixes an issue with the inheritance of attribute fixers

Added a missing lib/block-kit.rb file to allow bundler to auto-require the gem.

Initial release of block-kit.