Remove {prettycode} dependency in favour of cli::code_highlight()

[moodymudskipper]

Will be closed by #488 after Gabor merges r-lib/cli#727 cc @wurli

[moodymudskipper]

A new version of cli was released and the PR was not considered, this looks like it could take forever or even be rejected.

Better to just patch it in constructive itself. I think something like this is safe:

# In code
code_highlight <- NULL

# In .onLoad
code_highlight <- cli::code_highlight
environment(code_highlight) <- rlang::new_environment(
  data = list(getParseData = get_parse_data)
  parent = environment(cli::code_highlight)
)
code_highlight <<- code_highlight

Or a bit less efficient but without polluting .onLoad :

code_highlight_fixed <- function(...) {
  code_highlight <- cli::code_highlight
  environment(code_highlight) <- rlang::new_environment(
    data = list(getParseData = get_parse_data)
    parent = environment(cli::code_highlight)
  )
  code_highlight(...)
}

where get_parse_data() is the function defined in the PR but slightly modified to have the same args as getParseData() so no code using it will break.

We could also do something with rlang::local_bindings() but that's more invasive, we'll need to unlock bindings and I think CRAN doesn't like that.

We need the tests as well, we can copy them but use construct() rather than code_highlight()