use_targets() should assert {rstudioapi} namespace

[pat-s]

Prework

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Description

When using targets::use_targets() in a fresh {renv} project, {rstudioapi} namespace is not available.
AFAICS this only suppressed the final file opening of _targets.R in the editor but wanted to report it anyhow.

Reproducible example

> targets::use_targets()
✖ Install packages {{future}}, {{future.callr}}, and {{future.batchtools}} to allow use_targets() to configure tar_make_future() options.
• Writing file "_targets.R".
• Writing file "run.R".
• Writing file "run.sh".
• Modify '_targets.R'
Error in loadNamespace(x) : there is no package called ‘rstudioapi’

Expected result

Functionality is skipped without throwing an error or a warning is issued similar to the ones about the future pkgs.

Diagnostic information

targets 0.12.0

[wlandau]

I agree that the outcome is not desirable for the user, but I would prefer not to have targets get involved directly. use_targets() is simply calling usethis::edit_file(), the latter of which requires rstudioapi when invoked inside RStudio. Maybe the default behavior of usethis::edit_file() should be to fall back on its current behavior for terminal sessions: to open the user's default text editor. But this is outside the scope of targets.