Add experimentalFeatures to initialization options (#287)

* Add experimentalFeatures to initialization options

Introduce a feature flagging system for opt-in experimental features.
Clients can enable features via initializationOptions.experimentalFeatures
with granular per-feature control or an 'all' flag to enable everything.

First experimental feature: missingInputsQuickfix (for upcoming code actions)

expressions/src/features.ts

@@ -0,0 +1,66 @@
+/**
+ * Experimental feature flags.
+ *
+ * Individual feature flags take precedence over `all`.
+ * Example: { all: true, missingInputsQuickfix: false } enables all
+ * experimental features EXCEPT missingInputsQuickfix.
+ *
+ * When a feature graduates to stable, its flag becomes a no-op
+ * (the feature will be enabled regardless of the configuration value).
+ */
+export interface ExperimentalFeatures {
+  /**
+   * Enable all experimental features.
+   * Individual feature flags take precedence over this setting.
+   * @default false
+   */
+  all?: boolean;
+
+  /**
+   * Enable quickfix code action for missing required action inputs.
+   * @default false
+   */
+  missingInputsQuickfix?: boolean;
+}
+
+/**
+ * Keys of ExperimentalFeatures that represent actual features (excludes 'all')
+ */
+export type ExperimentalFeatureKey = Exclude<keyof ExperimentalFeatures, "all">;
+
+/**
+ * All known experimental feature keys.
+ * This list must be kept in sync with the ExperimentalFeatures interface.
+ */
+const allFeatureKeys: ExperimentalFeatureKey[] = ["missingInputsQuickfix"];
+
+export class FeatureFlags {
+  private readonly features: ExperimentalFeatures;
+
+  constructor(features?: ExperimentalFeatures) {
+    this.features = features ?? {};
+  }
+
+  /**
+   * Check if an experimental feature is enabled.
+   *
+   * Resolution order:
+   * 1. Explicit feature flag (if set)
+   * 2. `all` flag (if set)
+   * 3. false (default)
+   */
+  isEnabled(feature: ExperimentalFeatureKey): boolean {
+    const explicit = this.features[feature];
+    if (explicit !== undefined) {
+      return explicit;
+    }
+    return this.features.all ?? false;
+  }
+
+  /**
+   * Returns list of all enabled experimental features.
+   */
+  getEnabledFeatures(): ExperimentalFeatureKey[] {
+    return allFeatureKeys.filter(key => this.isEnabled(key));
+  }
+}