leadingTrivia at the file level is not actually leading

When declaring a new file with the `leadingTrivia` option, the comments are not added at the very top (= leading) section of the file.

Added imports will supersede the comments and comments will be bumped down.

Version: 20.0.0

**To Reproduce**

From [playground](https://awhitty.me/ts-morph-playground/#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).

```ts
import { Project, VariableDeclarationKind } from 'ts-morph';

export default function transform(project: Project) {
project.getSourceFile('index.ts').delete()

const f = project.createSourceFile('index.ts', {
leadingTrivia: '// trivia',
})

f.addImportDeclaration({
moduleSpecifier: 'vite',
namedImports: ['defineConfig'],
})

f.addVariableStatement({
declarationKind: VariableDeclarationKind.Const,
declarations: [
{
name: 'config',
initializer: '1',
},
],
})
};
```

Result:

```ts
import { defineConfig } from "vite";

// trivia
const config = 1;
```

**Expected behavior**

Comments (trivia) to be added to the top of the file.

Read CONTRIBUTING.md, then run the linked playground reproduction using createSourceFile with leadingTrivia followed by addImportDeclaration. Trace how the file-level trivia is positioned after the import is added, and consider the issue complete when the generated output keeps the comments at the very top of the file.

Written by the indexing model from the issue text.