Add cursor grab for web target (#2025)

* Add cursor grab

* Update feature matrix, changelog and platform information

* Add proper error propagation

* Remove "expect" from fallible code

    code would crash if handling pointer capture outside of winit on
    every mouse click

    we swallow the error since we could not think of a case where this
    would fail in a way that would want to handle that it fails

* Remove unnecessary implementation comment

Co-authored-by: Will Crichton <wcrichto@cs.stanford.edu>

src/platform_impl/web/web_sys/canvas.rs

@@ -89,6 +89,20 @@ impl Canvas {
         })
     }
 
+    pub fn set_cursor_grab(&self, grab: bool) -> Result<(), RootOE> {
+        if grab {
+            self.raw().request_pointer_lock();
+        } else {
+            let window = web_sys::window()
+                .ok_or(os_error!(OsError("Failed to obtain window".to_owned())))?;
+            let document = window
+                .document()
+                .ok_or(os_error!(OsError("Failed to obtain document".to_owned())))?;
+            document.exit_pointer_lock();
+        }
+        Ok(())
+    }
+
     pub fn set_attribute(&self, attribute: &str, value: &str) {
         self.common
             .raw