3.2.0 release

[eitsupi]

I've created https://github.com/REditorSupport/vscode-R/milestone/4

Since a lot of changes have been made, I think we could consider releasing version 3.1.0.

@renkun-ken @Fred-Wu
Do you have any thoughts on that?

[renkun-ken]

Agree

[Fred-Wu]

Agree

[eitsupi]

Thanks.

#1741 includes breaking changes to the sess package, so if we include #1741, I would like to remove deprecated functions from sess.

[eitsupi]

Related to #1751, maybe we should choose 3.2.0.