Escape special chars in encoding shorthand syntax

[brucala]

Due to the shorthand string syntax, using chars like ( or : in the Encoding can give unexpected results:

julia > Encoding("Beak Length (mm):Q")
Deneb.EncodingSpec: 
{
  "x": {
    "aggregate": "Beak Length ",
    "field": "mm",
    "type": "quantitative"
  }
}

At the moment there are no escaping rules to avoid that. In the meantime one can simply use the verbose syntax without the shorthands:

> julia> Encoding(x=(field="Beak Length (mm)", type=:quantitative))
Deneb.EncodingSpec: 
{
  "x": {
    "field": "Beak Length (mm)",
    "type": "quantitative"
  }
}