feat(style): compose generated class states

Add CssClass/CssClasses with/with_if helpers so nested partial view data can compose generated CSS constants without ad hoc class strings.

req: style/003

req: component/003

slhx-core/src/lib.rs

@@ -186,6 +186,14 @@ impl CssClass {
     pub const fn as_str(self) -> &'static str {
         self.name
     }
+
+    pub fn with(self, class: CssClass) -> CssClasses {
+        CssClasses::from([self, class])
+    }
+
+    pub fn with_if(self, condition: bool, class: CssClass) -> CssClasses {
+        CssClasses::from(self).with_if(condition, class)
+    }
 }
 
 impl AsRef<str> for CssClass {
@@ -315,6 +323,22 @@ impl CssClasses {
     pub fn as_str(&self) -> &str {
         &self.names
     }
+
+    pub fn with(mut self, class: CssClass) -> Self {
+        if !self.names.is_empty() {
+            self.names.push(' ');
+        }
+        self.names.push_str(class.as_str());
+        self
+    }
+
+    pub fn with_if(self, condition: bool, class: CssClass) -> Self {
+        if condition {
+            self.with(class)
+        } else {
+            self
+        }
+    }
 }
 
 impl From<CssClass> for CssClasses {