Name completion in pipe chains

[andycraig]

Currently the session watcher provides name completion for data frame names using $. It would be nice if this also worked for pipe chains:

mtcars %>%
    select(

One way to do this might be to provide the names as completion options any time there is a pipe and an unmatched ( between the start of the pipe chain and the cursor.

For now just having completions for the variable at the start of the pipe chain would be good. Functions like mutate can add new columns but I don't think we need to worry about having them as completion options initially.

[renkun-ken]

Would be nice to have this.

I implemented a document-wide token completion (REditorSupport/languageserver#324) in languageserver in an attempt to provide some partial support based on static analysis. If the columns do not appear as symbols anywhere in the document, then it does not work.

[andycraig]

Thanks for the reference @renkun-ken. Reading it I discovered that there is already an open issue for this, so I will close in favour of #323