feat(traces): add Funnel trace type

[jqnatividad]

What

Adds the Funnel trace type, plus the layout-level funnelmode/funnelgap/funnelgroupgap attributes and a FunnelMode enum.

The bundled plotly.min.js has registered funnel all along:

moduleType:"trace", name:"funnel",
categories:["bar-like","cartesian","svg","oriented","showLegend","zoomScale"]

so this is a pure Rust-wrapper addition — no JS work, no bundle bump. Because funnel is cartesian, it composes with subplot grids and with static image export.

Field set

Modelled on Bar, filtered to what plotly.js actually declares for funnel:

• offset and width are arrayOk: false in the schema, so they are scalar Option<f64> rather than Dim<f64>.
• No base, error bars, calendars, or point selection — funnel's supplyDefaults sets moduleHasSelected: false and the attribute block has no equivalents.
• textinfo is Option<String>. plotly.js takes a +-joined flaglist (label|text|percent initial|percent previous|percent total|value, plus none), and enumerating the combinations the way HoverInfo does would explode. This follows the existing precedent in pie.rs, which declares text_info: Option<String> and documents .text_info("label+percent"). Happy to switch to a typed flaglist if you'd prefer one.
• Connector is a new small struct (visible/line/fillcolor) in the same module; nothing equivalent existed, since waterfall is not implemented.

Layout attributes

funnelmode/funnelgap/funnelgroupgap are included alongside the existing waterfallmode/violinmode/boxmode fields. Without funnelmode, stacked and grouped multi-trace funnels are unreachable. This mirrors WaterfallMode, which already exists without a corresponding trace.

Out of scope

funnelarea is a different trace — domain-based and pie-like rather than cartesian — and is deliberately not included here.

Tests

• Doctest on Funnel asserting the serialized JSON.
• default_funnel, serialize_funnel (all fields), serialize_connector, and a horizontal-funnel-in-a-subplot case covering orientation, multi-flag textinfo, and xaxis/yaxis placement.
• serialize_funnel_mode in layout::modes.

cargo test -p plotly --features all, cargo clippy --features all -- -D warnings -A deprecated, and cargo +nightly fmt --all -- --check all pass.