No more double vars_select() 🚫

[juliasilge]

Before this PR, tidyselect::vars_select() was called twice inside of initial_split(). This generated messages like:

"Message"
"  Note: Using an external vector in selections is ambiguous."
"  i Use `all_of(group)` instead of `group` to silence this message."
"  i See <https://tidyselect.r-lib.org/reference/faq-external-vector.html>."
"  This message is displayed once per session."

We don't need to vars_select() twice, and instead can pass strata as {{ strata }}. Closes #294

[juliasilge]

This does in fact remove the message from the GH action runs ⬆️

[github-actions]

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