fix(hcl2cdk): Heredocs expression parsing

[mutahhir]

Related issue

Fixes #2753

Description

For string based expressions, we were wrapping them in quotes, and not checking if the expression was a here document. This PR fixes that oversight. Additionally, HCL expects a newline at the end of the HEREDOC trailing marker, so the code also force-adds a new line if it doesn't exist.

Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[ansgarm]

Nice 💯

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days. This helps our maintainers find and focus on the active issues. If you've found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.