Expose error and warning locations as data

[marcoroth]

This pull request adds the type and location of an error or warning as data, so a tool can point at the source without reading it out of the message with a regular expression.

Errors and warnings used to cross the FFI boundary as a single string, so the location and the kind of error were lost there. The Rust side now sends each one as JSON with a message, a type and a location, and the C extension builds the exception with from_json on the error class before raising it.

The shape follows Prism, Herb and Yerba. A warning is a LightningCSS::Warning with a message, a type and a location, like a diagnostic in Prism or Herb. A LightningCSS::Location is a Data with a filename, a line counted from 1 and a column counted from 0, like a location in all three.

css = ".a { color: red; }\n.b, { color: blue; }\n"

begin
  LightningCSS.transform(css, filename: "main.css")
rescue LightningCSS::ParseError => error
  error.message  #=> "Invalid empty selector at main.css:2:5"
  error.type     #=> "SelectorError"
  error.location #=> #<data LightningCSS::Location filename="main.css", line=2, column=4>
end

warning = LightningCSS.transform(css, filename: "main.css", error_recovery: true).warnings.first

warning.message  #=> "Invalid empty selector"
warning.type     #=> "SelectorError"
warning.location #=> #<data LightningCSS::Location filename="main.css", line=2, column=4>
warning.to_s     #=> "Invalid empty selector at main.css:2:5"

Resolves #18