feat(cli): support custom renderers via [renderer] config

[rlch]

Summary

Adds an optional [renderer] section to Dioxus.toml so projects using dioxus-core with a custom renderer can use dx serve, dx build, dx bundle, and dx fmt — without pulling in any built-in dioxus renderers.

Existing projects without this section are completely unaffected.

Why?

If you use dioxus-core for the VirtualDom/signals/hooks but bring your own rendering pipeline (winit+wgpu+vello, Bevy, etc.), dx currently can't help you — it expects a recognized renderer feature like web or native. This makes the detection configurable.

Example

[renderer]
name = "my-app"
default_platform = "desktop"

[renderer.features]
desktop = []
web = ["my-web-feature"]
ios = ["my-mobile"]
android = ["my-mobile"]

What changed

• RendererConfig struct in dioxus_config.rs — name, default_platform, features map with fallback chain lookup
• Each platform arm in request.rs checks custom config before the default dioxus feature injection
• Platform::from_identifier() made pub(crate) for config-driven resolution
• Schema updated
• Tests added

Backward compat

RendererConfig defaults to empty via #[serde(default)]. When absent, all code paths are unchanged.

Test plan

• cargo test -p dioxus-cli -- config::dioxus_config — 5 pass (3 existing + 2 new)
• cargo clippy -p dioxus-cli -- -D warnings — clean
• cargo fmt -p dioxus-cli -- --check — clean
• dx build / dx bundle work with a custom renderer project
• CI verifies existing examples still build

[rlch]

Tbh don't need this anymore because I ended up making my own CLI for my renderer, but might be useful for the community nonetheless. Feel free to close.