[VS Code] add setting for default language

[JosiahParry]

When using VS Code with Quarto inserting a code chunk always adds the code chunk type to python.

It would be desirable to be able to set a global or workspace setting to change the default code chunk type. This could be a scalar string value that would be used to populate the code chunk. For example, the setting could look like:

{
  "quarto.cells.defaultLanguage": "r"
}

Related: #170