provide rstudio-like keymaps

Describe the solution you'd like

I'd love it if there was a way to quickly get the key bindings that I (and I suspect many others) have muscle-memoried from RStudio.

Caveats:

• This should only cover R-specific keybindings, such as for devtools::load_all() or R CMD check and not the general-purpose IDE/Editor bindings (i.e. CTRL+1 for "move to source editor".
  VSCode is a different beast and has a completely different design, so it doesn't make sense for it to behave like RStudio all around.
  As a heuristic, only those keybindings should be enabled which only make sense inside R projects.
  If there's already a native, cross-language VSCode shortcut, that should take precedence.
• Consequently, this should only trigger when "when": "editorLangId == r || editorLangId == rmd && editorTextFocus".
• Some default VSCode keybindings would have to be overwritten (see below).
• The whole thing could maybe be enabled as an option in the setting.

Here's what I have so far (using tasks as per #59):

    {
        "description": "Insert <-",
        "key": "alt+-",
        "command": "type",
        "when": "editorLangId == r || editorLangId == rmd && editorTextFocus",
        "args": {
            "text": "<- "
        }
    },
    {
        "description": "Insert %>%",
        "key": "ctrl+shift+m",
        "command": "type",
        "when": "editorLangId == r || editorLangId == rmd && editorTextFocus",
        "args": {
            "text": "%>% "
        }
    },
    {
        // "description": "Disable (overloaded with above)",
        "key": "ctrl+shift+m",
        "when": "editorLangId == r || editorLangId == rmd && editorTextFocus",
        "command": "-workbench.actions.view.problems"
    },
    {
        "description": "Load all", 
        "key": "cmd+shift+l",
        "command": "r.runCommand",
        "when": "editorLangId == r || editorLangId == rmd",
        "args": "devtools::load_all()"
    },
    {
        "description": "Test",
        "key": "cmd+shift+t",
        "command": "workbench.action.tasks.runTask",
        "when": "editorLangId == r || editorLangId == rmd",
        "args": "Test"
    },
    {
        "description": "Document",
        "key": "cmd+shift+d",
        "command": "workbench.action.tasks.runTask",
        "when": "editorLangId == r || editorLangId == rmd",
        "args": "Document"
    },
    {
        "description": "Check",
        "key": "cmd+shift+e",
        "command": "workbench.action.tasks.runTask",
        "when": "editorLangId == r || editorLangId == rmd",
        "args": "Check"
    },

Start by tracing the existing r.runCommand and workbench.action.tasks.runTask integrations used by the proposed bindings. Compare the RStudio-specific shortcuts and their R/Rmd when conditions, then determine how an optional setting should enable them. Done means the requested R-specific mappings work without replacing native cross-language VS Code shortcuts.