fix: ensure abstraction type declaration is preserved

Author: Pavel910

.changeset/kind-singers-walk.md

@@ -0,0 +1,5 @@
+---
+"@webiny/di": patch
+---
+
+ensure generic type is preserved in Abstraction.d.ts when built

src/Abstraction.ts

@@ -9,8 +9,13 @@ type Implementation<A extends Abstraction<any>, I extends Constructor> = I & {
 
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
 export class Abstraction<T> {
-  // If the generic type is not used in any way, TS simply ignores it, thus breaking the desired type checking.
-  private readonly __type?: T;
+  /**
+   * If the generic type is not used in any way, TS simply ignores it, thus breaking the desired type checking.
+   * We must set this to `protected` to prevent TSC from stripping the type away.
+   * @internal
+   */
+  protected readonly __type!: T;
+
   public readonly token: symbol;
 
   constructor(name: string) {