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>

FEATURES.md

@@ -197,7 +197,7 @@ Legend:
 |----------------------- | -----    | ----   | ------- | ----------- | ----- | ----- | -------- |
 |Mouse events            |✔️       |▢[#63]  |✔️       |✔️          |**N/A**|**N/A**|✔️        |
 |Mouse set location      |✔️       |✔️      |✔️       |❓           |**N/A**|**N/A**|**N/A**|
-|Cursor grab             |✔️       |▢[#165] |▢[#242]  |✔️         |**N/A**|**N/A**|❓        |
+|Cursor grab             |✔️       |▢[#165] |▢[#242]  |✔️         |**N/A**|**N/A**|✔️        |
 |Cursor icon             |✔️       |✔️      |✔️       |✔️           |**N/A**|**N/A**|✔️        |
 |Touch events            |✔️       |❌      |✔️       |✔️          |✔️    |✔️     |❌        |
 |Touch pressure          |✔️       |❌      |❌       |❌          |❌    |✔️     |❌        |