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/window.rs

@@ -207,8 +207,11 @@ impl Window {
     }
 
     #[inline]
-    pub fn set_cursor_grab(&self, _grab: bool) -> Result<(), ExternalError> {
-        Err(ExternalError::NotSupported(NotSupportedError::new()))
+    pub fn set_cursor_grab(&self, grab: bool) -> Result<(), ExternalError> {
+        self.canvas
+            .borrow()
+            .set_cursor_grab(grab)
+            .map_err(|e| ExternalError::Os(e))
     }
 
     #[inline]