Auto merge of #20707 - gterzian:embedder_handling_of_prompts_and_alerts_2, r=payl

Embedder handling of prompts and alerts

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Allow embedder to allow alerts and prompts, but via constellation forwarding messages from script.

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach build-geckolib` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #19992  (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Cargo.lock

@@ -182,6 +182,7 @@ dependencies = [
  "bluetooth_traits 0.0.1",
  "compositing 0.0.1",
  "device 0.0.1 (git+https://github.com/servo/devices)",
+ "embedder_traits 0.0.1",
  "ipc-channel 0.10.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "servo_config 0.0.1",
@@ -419,6 +420,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 name = "compositing"
 version = "0.0.1"
 dependencies = [
+ "embedder_traits 0.0.1",
  "euclid 0.17.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "gfx_traits 0.0.1",
  "gleam 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -772,7 +774,14 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 name = "embedder_traits"
 version = "0.0.1"
 dependencies = [
+ "ipc-channel 0.10.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "log 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "msg 0.0.1",
+ "serde 1.0.37 (registry+https://github.com/rust-lang/crates.io-index)",
+ "servo_url 0.0.1",
+ "style_traits 0.0.1",
+ "webrender_api 0.57.2 (git+https://github.com/servo/webrender)",
 ]
 
 [[package]]
@@ -1847,7 +1856,6 @@ version = "0.0.1"
 dependencies = [
  "base64 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "brotli 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "compositing 0.0.1",
  "cookie 0.10.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "devtools_traits 0.0.1",
  "embedder_traits 0.0.1",
@@ -2593,6 +2601,7 @@ dependencies = [
  "canvas_traits 0.0.1",
  "cookie 0.10.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "devtools_traits 0.0.1",
+ "embedder_traits 0.0.1",
  "euclid 0.17.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "gfx_traits 0.0.1",
  "hyper 0.10.13 (registry+https://github.com/rust-lang/crates.io-index)",