feat(netwatch): Add browser support for rough network changes (online/offline) (#14)

## Description

Uses `window.navigator.onLine` and [`window.addEventListener("online",
...)` and
friends](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/onLine#listening_for_changes_in_network_status)
to listen for network changes.

## Notes & open questions

I tested this manually in firefox, chromium and gnome's WebKit-based
browser.
~~I'm not sure how easy it would be to set up automated testing for this
:S~~
Pretty hard. I've at least made sure there's a node.js smoke test for
this, so that *using* the API doesn't fail, even if the underlying
`navigator.onLine` or "online"/"offline" event listener APIs don't
exist.
Even if this were run in a browser, it'd be hard to trigger
online/offline.

The code generally falls back to not doing monitoring when the platform
doesn't support it (e.g. node.js). In that case, you just won't get any
events.

## Change checklist
- [x] Self-review.
- [x] Test

Author: matheus23

.cargo/config.toml

@@ -0,0 +1,3 @@
+[target.wasm32-unknown-unknown]
+runner = "wasm-bindgen-test-runner"
+rustflags = ['--cfg', 'getrandom_backend="wasm_js"']

.github/workflows/ci.yaml

@@ -147,6 +147,40 @@ jobs:
       env:
         RUST_LOG: ${{ runner.debug && 'TRACE' || 'DEBUG' }}
 
+  wasm_test:
+    name: Build & test wasm32 for browsers
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Add wasm target
+        run: rustup target add wasm32-unknown-unknown
+
+      - name: Install cargo-binstall
+        uses: cargo-bins/cargo-binstall@main
+      
+      - name: Install wasm-bindgen-test-runner
+        run: cargo binstall wasm-bindgen-cli --locked --no-confirm
+
+      - name: Install wasm-tools
+        uses: bytecodealliance/actions/wasm-tools/setup@v1
+
+      - name: wasm32 build (netwatch)
+        run: cargo build --target wasm32-unknown-unknown -p netwatch
+
+      # If the Wasm file contains any 'import "env"' declarations, then
+      # some non-Wasm-compatible code made it into the final code.
+      - name: Ensure no 'import "env"' in netwatch Wasm
+        run: |
+          ! wasm-tools print --skeleton target/wasm32-unknown-unknown/debug/netwatch.wasm | grep 'import "env"'
+
+      - name: Run smoke test in wasm
+        run: cargo test -p netwatch --test smoke --target=wasm32-unknown-unknown
+
   check_semver:
     runs-on: ubuntu-latest
     env: