Add initial no-tty implementation

[bulters]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

This closes #160

[meowgorithm]

Hi, and thanks for the contribution. Question: does this not already work headless? From the README:

pop < message.md \
    --from "me@example.com" \
    --to "you@example.com" \
    --subject "Hello, world!" \
    --attach invoice.pdf

[jficz]

I believe this (merged) PR solves the issue as well #124

If not, why exactly? iirc hasStdin() only tests for body input so it should be the only case where tty test happen(ed)