Helper script-writing functions with tidy eval for templating

[wlandau]

Prework

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Proposal

Reflecting on #290, #291, and #292, brew would be difficult to write _targets.R and other scripts for session-specific pipelines inside Shiny apps. In the next commit, I will add new functions tar_helper() and tar_helper_raw() to make it easier to write such scripts. tar_helper() uses tidy evaluation as a templating mechanism.

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