Always scan root for script templates in background

^KT-29893 Fixed

Provide AsyncScriptDefinitionsContributor.
Show a notification if the highlighting for script is requested when not all script templates are loaded

idea/idea-core/src/org/jetbrains/kotlin/idea/core/script/ScriptDefinitionsManager.kt

@@ -74,6 +74,7 @@ class ScriptDefinitionsManager(private val project: Project) : LazyScriptDefinit
 
     override fun findScriptDefinition(fileName: String): KotlinScriptDefinition? {
         if (nonScriptFileName(fileName)) return null
+        if (!isReady()) return null
 
         val cached = synchronized(scriptDefinitionsCacheLock) { scriptDefinitionsCache.get(fileName) }
         if (cached != null) return cached
@@ -144,6 +145,12 @@ class ScriptDefinitionsManager(private val project: Project) : LazyScriptDefinit
         }
     }
 
+    fun isReady(): Boolean {
+        return definitionsByContributor.keys.all { contributor ->
+            contributor.isReady()
+        }
+    }
+
     override fun getDefaultScriptDefinition(): KotlinScriptDefinition {
         val standardScriptDefinitionContributor = ScriptDefinitionContributor.find<StandardScriptDefinitionContributor>(project)
             ?: error("StandardScriptDefinitionContributor should be registered is plugin.xml")
@@ -271,6 +278,7 @@ interface ScriptDefinitionContributor {
     val id: String
 
     fun getDefinitions(): List<KotlinScriptDefinition>
+    fun isReady() = true
 
     companion object {
         val EP_NAME: ExtensionPointName<ScriptDefinitionContributor> =
@@ -328,4 +336,4 @@ class BundledKotlinScriptDependenciesResolver(private val project: Project) : De
             ?: PathUtilEx.getAnyJdk(project)
         return jdk?.homePath
     }
-}
+}