fix: renaming property does not remove old property (#157)

The following pseud-code would incorrectly result in a struct containing `newProp` and `oldProp`.

```ts
  struct.add({ name: 'oldProp' });
  struct.rename('oldProp', 'newProp');
```

src/builder/struct.ts

@@ -184,7 +184,7 @@ export class Struct implements IStructBuilder, HasProperties, HasFullyQualifiedN
     const old = this._properties.get(name);
 
     if (!old) {
-      throw `Unable top update property '${name}' in '${this._base.fqn}: Property does not exists, please use \`add\`.'`;
+      throw `Unable to update property '${name}' in '${this._base.fqn}: Property does not exists, please use \`add\`.'`;
     }
 
     const updatedProp = {
@@ -200,6 +200,10 @@ export class Struct implements IStructBuilder, HasProperties, HasFullyQualifiedN
       },
     };
 
+    if (updatedProp.name !== name) {
+      this.omit(name);
+    }
+
     return this.add(updatedProp);
   }
 

test/projen/projen-struct.test.ts

@@ -254,6 +254,30 @@ test('can updateAll props', () => {
   expect(renderedFile).toMatchSnapshot();
 });
 
+test('can rename a prop', () => {
+  // ARRANGE
+  const project = new TestProject();
+
+  // ACT
+  const struct = new ProjenStruct(project, { name: 'MyInterface' });
+  struct.add(
+    {
+      name: 'oldProp',
+      type: { primitive: PrimitiveType.Boolean },
+      optional: true,
+    },
+  );
+  struct.rename('oldProp', 'newProp');
+
+  // PREPARE
+  const renderedFile = synthSnapshot(project)['src/MyInterface.ts'];
+
+  // ASSERT
+  expect(renderedFile).not.toContain('oldProp');
+  expect(renderedFile).toContain('newProp?: boolean');
+  expect(renderedFile).toMatchSnapshot();
+});
+
 test('can import type from the same package at the top level', () => {
   // ARRANGE
   const project = new TestProject();