feat(dsl): allow setting output in Kotlin-based step (#1409)

Exposes a Kotlin API for setting step outputs like this:
`outputs["some-output"] = "foobar"`, accessible so far only via e.g.
`${{ steps.step-id.outputs.some-output }}`.

Author: krzema12

github-workflows-kt/api/github-workflows-kt.api

@@ -607,6 +607,7 @@ public final class io/github/typesafegithub/workflows/domain/contexts/Contexts {
 	public static synthetic fun copy$default (Lio/github/typesafegithub/workflows/domain/contexts/Contexts;Lio/github/typesafegithub/workflows/domain/contexts/GithubContext;ILjava/lang/Object;)Lio/github/typesafegithub/workflows/domain/contexts/Contexts;
 	public fun equals (Ljava/lang/Object;)Z
 	public final fun getGithub ()Lio/github/typesafegithub/workflows/domain/contexts/GithubContext;
+	public final fun getOutputs ()Lio/github/typesafegithub/workflows/domain/contexts/OutputsContext;
 	public fun hashCode ()I
 	public fun toString ()Ljava/lang/String;
 }
@@ -674,6 +675,11 @@ public final class io/github/typesafegithub/workflows/domain/contexts/GithubCont
 	public final fun serializer ()Lkotlinx/serialization/KSerializer;
 }
 
+public final class io/github/typesafegithub/workflows/domain/contexts/OutputsContext {
+	public static final field INSTANCE Lio/github/typesafegithub/workflows/domain/contexts/OutputsContext;
+	public final fun set (Ljava/lang/String;Ljava/lang/String;)V
+}
+
 public final class io/github/typesafegithub/workflows/domain/triggers/BranchProtectionRule : io/github/typesafegithub/workflows/domain/triggers/Trigger {
 	public static final field Companion Lio/github/typesafegithub/workflows/domain/triggers/BranchProtectionRule$Companion;
 	public fun <init> ()V

github-workflows-kt/src/main/kotlin/io/github/typesafegithub/workflows/domain/contexts/Contexts.kt

@@ -2,4 +2,6 @@ package io.github.typesafegithub.workflows.domain.contexts
 
 public data class Contexts(
     val github: GithubContext,
-)
+) {
+    val outputs: OutputsContext = OutputsContext
+}

github-workflows-kt/src/main/kotlin/io/github/typesafegithub/workflows/domain/contexts/OutputsContext.kt

@@ -0,0 +1,12 @@
+package io.github.typesafegithub.workflows.domain.contexts
+
+import java.io.File
+
+public object OutputsContext {
+    public operator fun set(
+        outputName: String,
+        value: String,
+    ) {
+        File(System.getenv("GITHUB_OUTPUT")).appendText("$outputName=$value")
+    }
+}