README: Open R terminal with VS Code command palette

[hturner]

Currently we say to start an R terminal by clicking "R:(not attached)" (https://github.com/r-devel/r-dev-env/blob/main/README.md?plain=1#L92-L95). This is fine when working through the README from start to finish, but if you skip the section on running R and have not opened an R file then "R:(not attached)" may not be shown.

So we can change this to open the R terminal through the command palette:

Ctrl/Cmd + Shift +P
type "R terminal"
select "R: Create R terminal"

[hturner]

I recently switched back to Firefox and this shortcut opens a private window rather than opening the command palette.

So now I have to use F1 instead. Maybe we need a short note on how to find the right shortcut (it is shown in the View menu, there may be another way).

[hturner]

We could add a footnote about this in the docs, not urgent.