Allow switching to source editor by document ID

[gadenbuie]

AFAICT it's currently not possible to activate a source editor by document ID, which makes unsaved documents inaccessible to automation when they lose focus.

library(rstudioapi)

documentNew("# first document")
id1 <- getSourceEditorContext()

# we have context for first document, open a second doc
documentNew("# second document")
id2 <- getSourceEditorContext()

# cannot re-activate first document

[gadenbuie]

I just followed a few internal Slack conversation threads and realized that rstudio/rstudio#8938 is intended to support a feature like this where the proposal was for

something like documentActivate(), that accepts either a document ID or a document path.