Update VS Code settings to use base R style indentation

[hturner]

R code in base R uses a tab display size of 8.

However, we also want pressing the tab key to mean indentation (with 4 spaces per indent), not tab character (8 spaces).

So the settings.json file should have:

"editor.tabSize": 8,
"editor.indentSize": 4
"editor.detectIndentation": false,

We need to set editor.detectIndentation to false to get tabSize/indentSize to work, see microsoft/vscode#155450 (comment).

The VS Code settings for the container are defined in .devcontainer/devcontainer.json.