Introduce a simple `SourceCode` component which offers simple APIs for generating source files.

Example:

```ts
const code = new SourceCode(project, 'source.ts');
code.line('// this is my source code');
code.open('export class Foo {');
code.open('public run() {');
code.line('console.log("running");');
code.close('}'); // method run
code.close('}'); // class Foo
```

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