DEV: Refactor subclasses in ThemeSettingsManager to individual files (#25605)

Why this change?

One Ruby class per file improves readability

lib/theme_settings_manager/string.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class ThemeSettingsManager::String < ThemeSettingsManager
+  def is_valid_value?(new_value)
+    (@opts[:min]..@opts[:max]).include? new_value.to_s.length
+  end
+
+  def textarea
+    @opts[:textarea]
+  end
+
+  def json_schema
+    begin
+      JSON.parse(@opts[:json_schema])
+    rescue StandardError
+      false
+    end
+  end
+end