Add ActiveRecord::Relation#table_name type

## Overview

`ActiveRecord::Relation#table_name` is delegated to the model class via `method_missing`, but it was not defined in the RBS type definitions. This causes Steep to report a `NoMethodError` when calling `table_name` on a Relation instance.

This PR adds the `table_name` method definition to `ActiveRecord::Relation` with a return type of `String`.

## References
- Source: `ActiveRecord::ModelSchema::ClassMethods#table_name`
  - https://api.rubyonrails.org/classes/ActiveRecord/ModelSchema/ClassMethods.html#method-i-table_name
  - https://github.com/rails/rails/blob/v7.0.8/activerecord/lib/active_record/model_schema.rb#L247-L250

Author: babashunya

gems/activerecord/6.0/activerecord.rbs

@@ -400,6 +400,8 @@ module ActiveRecord
     include Sanitization::ClassMethods
 
     def arel_table: () -> Arel::Table
+
+    def table_name: () -> String
   end
 end
 

gems/activerecord/7.2/_test/activerecord-7.2.rb

@@ -89,4 +89,6 @@ class Article < ApplicationRecord
   User.order(:id).unscope(:order)
   User.reorder(id: :asc)
   User.order(id: :asc).reorder(id: :desc)
+
+  User.all.table_name
 end

gems/activerecord/7.2/activerecord.rbs

@@ -407,6 +407,8 @@ module ActiveRecord
     include Sanitization::ClassMethods
 
     def arel_table: () -> Arel::Table
+
+    def table_name: () -> String
   end
 end
 

gems/activerecord/8.0/_test/activerecord-8.0.rb

@@ -89,4 +89,6 @@ class Article < ApplicationRecord
   User.order(:id).unscope(:order)
   User.reorder(id: :asc)
   User.order(id: :asc).reorder(id: :desc)
+
+  User.all.table_name
 end

gems/activerecord/8.0/activerecord.rbs

@@ -407,6 +407,8 @@ module ActiveRecord
     include Sanitization::ClassMethods
 
     def arel_table: () -> Arel::Table
+
+    def table_name: () -> String
   end
 end