Fix pack chart labels using xfun::js() for JavaScript callbacks

[Copilot]

G2's pack mark transforms tree data into d3-hierarchy nodes, nesting original fields under d.data.name — inaccessible via plain JSON field strings. The fix uses xfun::js(), which xfun::tojson() serializes as a raw JS expression (unquoted), enabling callback functions in the G2 spec.

• examples/marks.Rmd: Replace broken labels = list(list(text = 'value')) with style.labelText JS callback per G2 official docs; add color = 'depth' encoding for visual depth cues

g2() |>
  mark_pack(
    data = list(type = 'inline', value = tree_data),
    encode = list(value = 'value', color = 'depth'),
    style = list(
      labelText = xfun::js(
        '(d) => d.r >= 10 && d.height === 0 ? d.data.name : ""'
      ),
      labelFontSize = 8
    ),
    legend = FALSE
  )

xfun::js() is the general pattern for any G2 property requiring a JS function (dynamic styles, tooltip titles, conditional visibility, etc.).