WIP R Executable Selection

[ElianHugh]

PR: Executable Selection

• Contributes: R executable selection.
• Related issues Create or provide choice for multiple R versions and x64 or i386 architectures for R terminal #696 R and radian paths per project folder/conda environment (i.e. make vscode-R design like vscode-python) #1130 Folder specific R path #1148 Can't start with the R in conda environment #747

Aims

1. Consolidate R path selection into a more user-friendly UI
2. Make it easier for users to find and use their R installations
3. Inform users when their R installations are not usable
4. Do not break any user workflows. The PR should be a seamless
   integration with what users are used to
5. Fully support virtual R installations (mamba and conda)

Features

New Status Bar

If there is no valid rpath set by the user, the language status item
will prompt the user to select an R executable.

The status item can be collapsed if the user doesn’t want the item to be
visible at all times.

Executable Selector

Users will be shown a quickpick of valid R paths that were either found
by VSCode, or were set by the user. Selecting a path will set the
executable to the current workspace.

The quickpick is aware of renv lock files and will attempt to recommend
R versions that match the R version in the lock file.

Conda/Mamba Support

The executable service is aware of conda and mamba installations, and
will modify any background services to use conda run or mamba run,
and will modify the terminal accordingly as well. Both conda and mamba
should work without any user intervention.

Multi-root workspace support

Different roots can have different R versions set to them.

If the user is working in a multi-root workspace, selecting an
executable will prompt the user to select which workspace they want the
executable to be set to.

[Billsfriend]

Thanks for the work! These features will greatly improve my experience, and I see a lot of issue actually related to this problem.