Multiline vars replacement break indentation in config generation

[yleflour]

Current situation

If the promptfile contains this:

- role: user
  content: |
    The file content is:
    ```
    {{fileContent}}
    ```

And the configfile contains this:

tests:
  - vars:
      fileContent: |
        // Sample comment
        export const hello = "word"

The resulting output is:

- role: user
  content: |
    The file content is:
    ```
    // Sample comment
export const hello = "word"
\```

Which is invalid

Expect result

A multiline var value should be indented as well:

- role: user
  content: |
    The file content is:
    ```
    // Sample comment
    export const hello = "word"
    ```