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

Why this change?

One Ruby class per file improves readability

lib/theme_settings_manager/float.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class ThemeSettingsManager::Float < ThemeSettingsManager
+  def self.cast(value)
+    value.to_f
+  end
+
+  def value
+    self.class.cast(super)
+  end
+
+  def value=(new_value)
+    super(self.class.cast(new_value))
+  end
+
+  def is_valid_value?(new_value)
+    (@opts[:min]..@opts[:max]).include? new_value.to_f
+  end
+end