Support resolving i18n message with standard way (#6648)

#### What type of PR is this?

/kind feature
/area theme
/sig docs
/milestone 2.20.x

#### What this PR does / why we need it:

After this PR, we can define i18n message files next to the template file.

```yaml
i18n:
    default.properties
templates:
    index.html
    index.properties # Higher properties than default.properties
    index_zh.properties # Higher properties than index.properties
    index_zh_CN.properties # Higher priority than index_zh.properties
```

It's convenient for plugins that define the template files.

See https://www.thymeleaf.org/doc/tutorials/3.1/usingthymeleaf.html#standard-message-resolver for more.

#### Does this PR introduce a user-facing change?

```release-note
支持在主题中通过 Thymeleaf 默认行为实现国际化
```

application/src/main/java/run/halo/app/theme/message/ThemeMessageResolver.java

@@ -1,7 +1,10 @@
 package run.halo.app.theme.message;
 
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 import org.thymeleaf.messageresolver.StandardMessageResolver;
 import org.thymeleaf.templateresource.ITemplateResource;
 import run.halo.app.theme.ThemeContext;
@@ -22,7 +25,12 @@ public class ThemeMessageResolver extends StandardMessageResolver {
     protected Map<String, String> resolveMessagesForTemplate(String template,
         ITemplateResource templateResource,
         Locale locale) {
-        return ThemeMessageResolutionUtils.resolveMessagesForTemplate(locale, theme);
+        var properties = new HashMap<String, String>();
+        Optional.ofNullable(ThemeMessageResolutionUtils.resolveMessagesForTemplate(locale, theme))
+            .ifPresent(properties::putAll);
+        Optional.ofNullable(super.resolveMessagesForTemplate(template, templateResource, locale))
+            .ifPresent(properties::putAll);
+        return Collections.unmodifiableMap(properties);
     }
 
     @Override

application/src/test/java/run/halo/app/theme/message/ThemeMessageResolutionUtilsTest.java

@@ -29,16 +29,16 @@ class ThemeMessageResolutionUtilsTest {
     void resolveMessagesForTemplateForDefault() throws URISyntaxException {
         Map<String, String> properties =
             ThemeMessageResolutionUtils.resolveMessagesForTemplate(Locale.CHINESE, getTheme());
-        assertThat(properties).hasSize(1);
-        assertThat(properties).containsEntry("index.welcome", "欢迎来到首页");
+        assertThat(properties).isEqualTo(Map.of("index.welcome", "欢迎来到首页",
+            "title", "来自 i18n/zh.properties 的标题"));
     }
 
     @Test
     void resolveMessagesForTemplateForEnglish() throws URISyntaxException {
         Map<String, String> properties =
             ThemeMessageResolutionUtils.resolveMessagesForTemplate(Locale.ENGLISH, getTheme());
-        assertThat(properties).hasSize(1);
-        assertThat(properties).containsEntry("index.welcome", "Welcome to the index");
+        assertThat(properties).isEqualTo(Map.of("index.welcome", "Welcome to the index",
+            "title", "这是来自 i18n/default.properties 的标题"));
     }
 
     @Test

application/src/test/java/run/halo/app/theme/message/ThemeMessageResolverIntegrationTest.java

@@ -93,6 +93,9 @@ public class ThemeMessageResolverIntegrationTest {
             .expectStatus()
             .isOk()
             .expectBody()
+            // make sure the "templates/index.properties" file is precedence over the
+            // "i18n/default.properties".
+            .xpath("/html/head/title").isEqualTo("Title from index.properties")
             .xpath("/html/body/div[1]").isEqualTo("foo")
             .xpath("/html/body/div[2]").isEqualTo("欢迎来到首页");
     }
@@ -105,7 +108,7 @@ public class ThemeMessageResolverIntegrationTest {
             .expectStatus()
             .isOk()
             .expectBody()
-            .xpath("/html/head/title").isEqualTo("Title")
+            .xpath("/html/head/title").isEqualTo("来自 index_zh.properties 的标题")
             .xpath("/html/body/div[1]").isEqualTo("zh")
             .xpath("/html/body/div[2]").isEqualTo("欢迎来到首页")
         ;

application/src/test/resources/themes/default/i18n/default.properties

@@ -1 +1,2 @@
-index.welcome=\u6B22\u8FCE\u6765\u5230\u9996\u9875
+index.welcome=欢迎来到首页
+title=这是来自 i18n/default.properties 的标题

application/src/test/resources/themes/default/i18n/zh.properties

@@ -0,0 +1 @@
+title=来自 i18n/zh.properties 的标题

application/src/test/resources/themes/default/templates/index.html

@@ -2,7 +2,7 @@
 <html lang="en">
 <head>
     <meta charset="UTF-8"/>
-    <title>Title</title>
+    <title th:text="#{title}">Title</title>
 </head>
 <body>
 index

application/src/test/resources/themes/default/templates/index.properties

@@ -0,0 +1 @@
+title=Title from index.properties

application/src/test/resources/themes/default/templates/index_zh.properties

@@ -0,0 +1 @@
+title=来自 index_zh.properties 的标题