Prompt and command to install `languageserver`

[renkun-ken]

vscode-R now requires languageserver to be installed.

If the language server fails to start due to missing languageserver package, it would make sense to prompt to user and ask whether to install the package for the user.

[renkun-ken]

To be clear,

• R session watcher only requires jsonlite and rlang as before.
• R language service requires languageserver which imports jsonlite and rlang.

[ManuelHentschel]

A related topic is pandoc/rmarkdown for the help-hover of the languageserver. Currently, the languageserver just serves the "old" layout if the required dependencies are not installed.

[hongooi73]

I agree that prompting for languageserver would be good. The current experience is rather unfriendly as the server just throws an error, which is confusing for new users.