Add WebGL browser globals

Specification: https://www.khronos.org/registry/webgl/specs/1.0/

These globals can be useful for doing things like this:

    var gl = document.getElementById("myCanvas").getContext("webgl");
    
    gl instanceof WebGLRenderingContext;                       // true
    gl.createBuffer() instanceof WebGLBuffer;                  // true
    gl.createFramebuffer() instanceof WebGLFramebuffer;        // true
    gl.createProgram() instanceof WebGLProgram;                // true
    gl.createRenderbuffer() instanceof WebGLRenderbuffer;      // true
    gl.createShader(gl.VERTEX_SHADER) instanceof WebGLShader;  // true
    gl.createTexture() instanceof WebGLTexture;                // true
Only need `WebGLContextEvent` https://www.khronos.org/registry/webgl/specs/1.0/#WEBGLCONTEXTEVENT