feat: use projen native TextFile features for generated files

[hoegertn]

• Please check if the PR fulfills these requirements

• The commit message describes your change
• Tests for the changes have been added if possible (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Changes are mentioned in the changelog (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  cdk-serverless now uses projen's native feature of TextFile components instead of writing files to disk manually

• What is the current behavior? (You can also link to an open issue here)
  cdk-serverless uses fs.writeFileSync to write generated files

• What is the new behavior (if this is a feature change)?
  cdk-serverless uses pj.TextFile to generate files with the correct lifecycle

• Does this PR introduce a breaking change? (What changes might users need to make in their setup due to this PR?)

• Other information:

[hoegertn]

@DerDackel as mentioned I move the file generation to projen's native components.

No idea why I did not do this before.

[DerDackel]

LGTM