prompt to install languageserver is not available

[shrektan]

What problem did you solve?

Fixes #754

Newbies may find it confusing when the R package languageserver is not installed. This PR enables a prompt to instruct the user to install languageserver if missing.

(We can use the added function to check other required R packages as well)
(I've also verified that it works on Windows)

How can I check this pull request?

1. (R) remove the package via remove.packages("languageserver")
2. Open VSCode with any R project
3. A prompt will show up. Click "Not sure" the message vanishes. Click "Click to install" will launch a task to install the package with a notice that the user may need to reload to take effect.

Screenshot

[renkun-ken]

@shrektan I rewrite some code in this PR:

• Checking languageserver installation at createClient to respect the working directory to be used (because different workspace folder might have different .Rprofile, e.g. renv might be used). Also, after installing languageserver, reopening an R file could activate the language service again.
• executeRCommand uses spawnSync instead of execSync to better handle the quoting of the arguments.
• executeRCommand now handles the case where user writes anything to stdout before and after the supplied expression is evaluated.

[shrektan]

@renkun-ken Looks good to me. Much cleaner. Thanks!