refactor: split theme_plugin.rs into a theme module

Move each type into its own file under `crates/egui/src/theme/`:
`StyleProvider`, `ThemeCache`, `DefaultStyle`, and `Themes`.
The module is renamed `egui::theme_plugin` -> `egui::theme`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

examples/styling_engine/src/custom_engine.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 
 use eframe::egui::{
     Color32,
-    theme_plugin::StyleProvider,
+    theme::StyleProvider,
     widget_style::{BaseStyle, ButtonStyle, HasClasses as _, StyleArgs},
 };
 use logos::Logos;

examples/styling_engine/src/main.rs

@@ -3,7 +3,7 @@
 
 use eframe::egui::{
     self, Button, Frame, Margin, Panel, UiBuilder,
-    theme_plugin::ThemeCache,
+    theme::ThemeCache,
     widget_style::{ButtonStyle, HasClasses as _},
 };