Conditionally propagate web events using a filter in WebOptions (#5056)

Currently egui will prevent all web events from propagating. This causes
issues in contexts where you are using egui in a larger web context that
wants to receive events that egui does not directly respond to. For
example, currently using egui in a VSCode extension will block all app
hotkeys, such as saving and opening the panel.

This adds a closure to `WebOptions` that takes in a reference to the
egui event that is generated from a web event and returns if the
corresponding web event should be propagated or not. The default for it
is to always return false.

Alternatives I considered were:
1. Having the propagation filter be a property of the focus in memory.
That way it could be configured by the view currently selected. I opted
away from that because I wanted to avoid lowering eframe implementation
specific stuff into egui.
2. Having events contain a `web_propagate` flag that could be set when
handling them. However, that would not be compatible with the current
system of egui events being handled outside of the web event handler.

I just recently started using egui so I am not sure how idiomatic my
approach here is. I would be happy to switch this over to a different
architecture if there are suggestions.

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crates/eframe/src/epi.rs

@@ -457,6 +457,14 @@ pub struct WebOptions {
     ///
     /// Defaults to true.
     pub dithering: bool,
+
+    /// If the web event corresponding to an egui event should be propagated
+    /// to the rest of the web page.
+    ///
+    /// The default is `false`, meaning
+    /// [`stopPropagation`](https://developer.mozilla.org/en-US/docs/Web/API/Event/stopPropagation)
+    /// is called on every event.
+    pub should_propagate_event: Box<dyn Fn(&egui::Event) -> bool>,
 }
 
 #[cfg(target_arch = "wasm32")]
@@ -472,6 +480,8 @@ impl Default for WebOptions {
             wgpu_options: egui_wgpu::WgpuConfiguration::default(),
 
             dithering: true,
+
+            should_propagate_event: Box::new(|_| false),
         }
     }
 }