fix(aws-ecs): fix use of published NPM package with TypeScript (#1117)

The package previously published would not work with a TypeScript consumer, because the generated `.d.ts` file was missing an import statement due to a bug in the TypeScript compiler. Add an explicit type declaration to work around this issue until it is fixed and released upstream.

Reference: microsoft/TypeScript#26969

packages/@aws-cdk/aws-ecs/lib/fargate/fargate-task-definition.ts

@@ -46,7 +46,10 @@ export class FargateTaskDefinition extends TaskDefinition {
   /**
    * The configured network mode
    */
-  public readonly networkMode = NetworkMode.AwsVpc;
+  public readonly networkMode: NetworkMode = NetworkMode.AwsVpc;
+  // NOTE: Until the fix to https://github.com/Microsoft/TypeScript/issues/26969 gets released,
+  // we need to explicitly write the type here, as type deduction for enums won't lead to
+  // the import being generated in the .d.ts file.
 
   constructor(parent: cdk.Construct, name: string, props: FargateTaskDefinitionProps = {}) {
     super(parent, name, {