fix(@ngtools/webpack): elide imports for implements keywords

Running the `remove-ivy-jit-support-calls` and `remove_decorators` transformers causes the following TS bug microsoft/TypeScript#17552 which is why the `elide-imports` transformer exists in the first place.

However, when having a syntax like the below;
```ts
import { AccountComponentChild } from '../@types';
export class SignUpComponent implements AccountComponentChild{}
```

The `implements` parts of the class is called a `HeritageClause` with child statements of `ExpressionWithTypeArguments` also the same is for `abstract`. With this change we check the token of the `HeritageClause` and if it's an `ImplementsKeyword` we elide the import.

Closes #16907

packages/ngtools/webpack/src/transformers/elide_imports.ts

@@ -39,6 +39,13 @@ export function elideImports(
       return;
     }
 
+    // Consider types for 'implements' as unused.
+    // A HeritageClause token can also be an 'AbstractKeyword'
+    // which in that case we should not elide the import.
+    if (ts.isHeritageClause(node) && node.token === ts.SyntaxKind.ImplementsKeyword) {
+      return;
+    }
+
     // Record import and skip
     if (ts.isImportDeclaration(node)) {
       imports.push(node);

packages/ngtools/webpack/src/transformers/elide_imports_spec.ts

@@ -317,6 +317,27 @@ describe('@ngtools/webpack transformers', () => {
       });
     });
 
+    it(`should remove import for 'ExpressionWithTypeArguments' implements token`, () => {
+      const input = tags.stripIndent`
+        import { Bar, Buz, Unused } from './bar';
+
+        export class Foo extends Bar implements Buz { }
+
+        ${dummyNode}
+      `;
+
+      const output = tags.stripIndent`
+        import { Bar } from './bar';
+
+        export class Foo extends Bar { }
+      `;
+
+      const { program, compilerHost } = createTypescriptContext(input);
+      const result = transformTypescript(undefined, [transformer(program)], program, compilerHost);
+
+      expect(tags.oneLine`${result}`).toEqual(tags.oneLine`${output}`);
+    });
+
     describe('should not elide imports decorator type references when emitDecoratorMetadata is true', () => {
       const extraCompilerOptions: ts.CompilerOptions = {
          emitDecoratorMetadata: true,