Use RStudio config file to pass theme and other useful parameters instead of R hooks

[avouacr]

We should transition from this to this to be cleaner and consistent with other IDEs. And maybe provide some other relevant defaults ?

Here is a user example :

{
    "editor_theme": "PaperColor3b",
    "posix_terminal_shell": "bash",
    "server_editor_font": "Source Code Pro",
    "server_editor_font_enabled": true,
    "save_workspace": "never",
    "load_workspace": false,
    "remove_history_duplicates": true,
    "auto_detect_indentation": true,
    "highlight_selected_line": true,
    "relative_line_numbers": true,
    "show_invisibles": true,
    "highlight_r_function_calls": true,
    "rainbow_parentheses": true,
    "auto_append_newline": true,
    "strip_trailing_whitespace": true,
    "check_unexpected_assignment_in_function_call": true,
    "style_diagnostics": true,
    "default_encoding": "UTF-8",
    "indent_guides": "gray",
    "syntax_color_console": true,
    "show_panel_focus_rectangle": true,
    "editor_keybindings": "vim"
}