Support rstudioapi::getThemeInfo()

[gadenbuie]

rstudioapi::getThemeInfo() was not included in the initial list of shimmed rstudioapi functions in #1312.

It'd be nice to support this function, in particular because it's used by thematic and would enable automatic theming of plots to reflect the current Positron theme.

Here's the info currently available in RStudio

rstudioapi::getThemeInfo()
#> $editor
#> [1] "Textmate (default)"
#> 
#> $global
#> [1] "Modern"
#> 
#> $dark
#> [1] FALSE
#> 
#> $foreground
#> [1] "rgb(0, 0, 0)"
#> 
#> $background
#> [1] "rgb(255, 255, 255)"

In particular, the key bits that I'd like to see in Positron:

1. foreground
2. background
3. dark (would be nice)
4. editor (would be nice)

I don't think we'd need the global item, or that could be "Positron".

[jennybc]

I just moved this into release candidate, because I'm about to make a change in reprex that won't fully take effect in Positron, until we offer rstudioapi::getThemeInfo().

In reprex's case, it's dark (TRUE / FALSE) that is really important, so a bit different from @gadenbuie's priorities.

We discussed this briefly in the R sync today, so I'll leave a few notes for whoever picks this up:

• We suspect returning "Modern" for the global theme is the right call. It is a concept that doesn't really map over to VS Code / Positron, so this is going to be somewhat artificial.
• Implementation will probably look a lot like the existing implementation of evaluate_when_clause in the ui comm.