web: make auto-css experimental (#1294)

Co-authored-by: Oleksandr Karpovich <oleksandr.karpovich@jetbrains.com>

web/core/src/jsMain/kotlin/org/jetbrains/compose/web/ExperimentalComposeWebStyleApi.kt

@@ -0,0 +1,4 @@
+package org.jetbrains.compose.web
+
+@RequiresOptIn("This API is experimental and is likely to change in the future.")
+annotation class ExperimentalComposeWebStyleApi

web/core/src/jsMain/kotlin/org/jetbrains/compose/web/css/StyleSheet.kt

@@ -4,6 +4,7 @@ import androidx.compose.runtime.Composable
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.setValue
+import org.jetbrains.compose.web.ExperimentalComposeWebStyleApi
 import org.jetbrains.compose.web.css.selectors.CSSSelector
 import org.jetbrains.compose.web.css.selectors.className
 import org.jetbrains.compose.web.dom.Style
@@ -86,6 +87,7 @@ open class StyleSheet(
     }
 
     // TODO: just proof of concept, do not use it
+    @ExperimentalComposeWebStyleApi
     fun css(cssBuild: CSSBuilder.() -> Unit): String {
         val selfSelector = CSSSelfSelector()
         val (style, newCssRules) = buildCSS(selfSelector, selfSelector, cssBuild)