Avoid space in #> when at start of line

[lorenzwalthert]

As noted through applying styler to a lot of Rmd in tidyverse/tidyverse.org#489, I realised we probably don't want to add a space inside #> if it's at the beginning of a line:

# in Rmd, when you insert the output to avoid having it to run later because of compatibility:
f()
#> x

{styler} currently does. See also #700.