Add a newline before appending the avo tailwind command to Procfile.dev

[cmaxw]

Description

When running the tailwind generator, it appends the avo command to the end of the file. If the file doesn't end in a newline, it adds it to the previous line.

Fixes # 3187

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Screenshots & recording

Manual review steps

1. Step 1
2. Step 2

Manual reviewer: please leave a comment with output from the test if that's the case.

[codeclimate]

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[adrianthedev]

Awesome! 🙌