Improve getContext2D binding

Author: nojaf

src/DOMAPI.res

@@ -9354,6 +9354,16 @@ type domPoint = {
   ...domPointReadOnly,
 }
 
+/**
+ [Read more on MDN](https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext#contextattributes)
+ */
+type canvasContext2DAttributes = {
+  alpha: bool,
+  colorspace?: predefinedColorSpace,
+  desynchronized: bool,
+  willReadFrequently: bool,
+}
+
 /**
 The CanvasRenderingContext2D interface, part of the Canvas API, provides the 2D rendering context for the drawing surface of a <canvas> element. It is used for drawing shapes, text, images, and other objects.
 [See CanvasRenderingContext2D on MDN](https://developer.mozilla.org/docs/Web/API/CanvasRenderingContext2D)

src/DOMAPI/HTMLCanvasElement.res

@@ -696,14 +696,15 @@ external togglePopover: (htmlCanvasElement, ~force: bool=?) => bool = "togglePop
 
 /**
 Returns an object that provides methods and properties for drawing and manipulating images and graphics on a canvas element in a document. A context object includes information about colors, line widths, fonts, and other graphic parameters that can be drawn on a canvas.
-@param contextId The identifier (ID) of the type of canvas to create. Internet Explorer 9 and Internet Explorer 10 support only a 2-D context using canvas.getContext("2d"); IE11 Preview also supports 3-D or WebGL context using canvas.getContext("experimental-webgl");
-[Read more on MDN](https://developer.mozilla.org/docs/Web/API/HTMLCanvasElement/getContext)
+Creates a CanvasRenderingContext2D object representing a two-dimensional rendering context.
+
+[Read more on MDN](https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext#2d)
 */
 @send
-external getContext: (
+external getContext2D: (
   htmlCanvasElement,
-  ~contextId: string,
-  ~options: JSON.t=?,
+  @as("2d") _,
+  ~options: canvasRenderingContext2D=?,
 ) => canvasRenderingContext2D = "getContext"
 
 /**

tests/DOMAPI/HTMLCanvasInputElement__tes.res

@@ -1,8 +1,8 @@
-open Global
+open WebAPI.Global
 
 let myCanvas: DOMAPI.htmlCanvasElement =
   document->Document.getElementById("myCanvas")->Prelude.unsafeConversation
-let ctx = myCanvas->HTMLCanvasElement.getContext(~contextId="2d")
+let ctx = myCanvas->HTMLCanvasElement.getContext2D
 
 ctx.fillStyle = Prelude.unsafeConversation("red")
 ctx->CanvasRenderingContext2D.fillRect(~x=50., ~y=50., ~w=200., ~h=200.)