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/web/app_runner.rs

@@ -6,7 +6,7 @@ use super::{now_sec, text_agent::TextAgent, web_painter::WebPainter, NeedRepaint
 
 pub struct AppRunner {
     #[allow(dead_code)]
-    web_options: crate::WebOptions,
+    pub(crate) web_options: crate::WebOptions,
     pub(crate) frame: epi::Frame,
     egui_ctx: egui::Context,
     painter: super::ActiveWebPainter,