egui_extras: Make serde an optional feature (#4641)

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All the other crates in egui have serde as an optional dependency -
which is great! But sadly egui_extras unconditionally includes it, which
adds a bunch of code to stuff that may not care for it. This PR gates
serde support behind a new `serde` feature.

This is a breaking change; if that's undesirable then we can add it as a
default feature instead, though that wouldn't match any of the other
crates.

crates/egui_demo_app/Cargo.toml

@@ -26,7 +26,7 @@ web_app = ["http", "persistence"]
 
 http = ["ehttp", "image", "poll-promise", "egui_extras/image"]
 image_viewer = ["image", "egui_extras/all_loaders", "rfd"]
-persistence = ["eframe/persistence", "egui/persistence", "serde"]
+persistence = ["eframe/persistence", "egui/persistence", "serde", "egui_extras/serde"]
 puffin = ["eframe/puffin", "dep:puffin", "dep:puffin_http"]
 serde = ["dep:serde", "egui_demo_lib/serde", "egui/serde"]
 syntect = ["egui_demo_lib/syntect"]