Update R Docker image and vscode-r recommended settings

[eitsupi]

This pull request updates R to latest version and add vscode-R recommended settings.

• Change base image rocker/r-apt:bionic(R 3.6.3) to rocker/r-ver, which provide specific version R (4.0.0 ~ 4.0.4) based on Ubuntu LTS.
• Change not to install libzip-dev. (libzip-dev was needed for R Tools Add R devcontainer for statistical computing #60, which is not included by Update rust, bazel, perl, azure-cli, R definitions to use common scripts #473 ).
• Install libxt-dev, for to plot without warning.
• Add vscode-R recommended settings and packages.
  • Install R LSP Client and R Language Server.
    • Install libcurl4-openssl-dev, libssl-dev, libxml2-dev, for to install R Language Server.
  • Install devtools
    • Install libgit2-dev, for to install devtools
  • Install radian.
    • Install python3-pip, for to install radian.
• Add vscode-R recommended settings.

[ghost]

All CLA requirements met.

[eitsupi]

@Chuxel I apologize for including the fix of username acaba59 in this PR.
You've changed username to docker in e2480ab, but I think vscode is more better username because this is other container's default username.
So, I have changed the username to vscode again c1548b4.

[2percentsilk]

@kmehant as the R definition maintainer, can I get your eyes on this? 👀

[eitsupi]

@2percentsilk @Chuxel @kmehant If this review is going to take a long time, would it be better to break the PR into base image update and the others?

[kmehant]

@eitsupi Looks good to me! However, I request a review from @Chuxel.
It would be great if you could squash commits wherever possible.

@2percentsilk Thanks for tagging me, I apologize for the late response.

[Chuxel]

LGTM! The new extension is low volume which gave me pause, but I see its maintained by the same maintainers as the existing extension. So from that perspective it's probably ok.

Made one fix to move the remoteUser back to vscode given this is what the dockerfile does.

[Chuxel]

@eitsupi @kmehant Actually, one thing I just noticed - the # [Choice] comment in the Dockerfile also can drive VS Code UX, but it has to be a concrete list.

At the moment I've removed the comment since I'm not sure if there's higher level tags like "4" and the lower level versions are going to be a bit of a maintenance issue.

[eitsupi]

@Chuxel Thank you for merging and the comment fix.

There is no higher level tags like "4", and minor versions like "4.0.4" will increase every few months.
So if there is no problem with the current state, I think it's fine the current state which only removed the # [Choice] comment.