feat: switch to native Promises for async APIs

- All async APIs now use native Promises -- no more Task or
CancellablePromise
- The cancellation API is mostly new. Command chains still provide a
`cancel` method. Tests, suites, and exector runs can be cancelled by
calling `cancel` on the test, suite, or executor.

references #1018

BREAKING CHANGE: async APIs now return native Promises, and
the cancellation API is entirely new

.eslintrc.json

@@ -28,6 +28,11 @@
     // TODO: turn this back on when we're ready to fix all the usages
     "@typescript-eslint/no-non-null-assertion": "off",
     // The whole point of ts-ignore is to purposefully ignore errors...
-    "@typescript-eslint/ban-ts-ignore": "off"
+    "@typescript-eslint/ban-ts-ignore": "off",
+    // It's OK to return values and ignore them
+    "@typescript-eslint/no-misused-promises": [
+      "error",
+      { "checksVoidReturn": false }
+    ]
   }
 }

src/common/index.ts

@@ -1,5 +1,4 @@
-export { default as Task } from './lib/Task';
-export * from './lib/Task';
+export * from './lib/cancel';
 
 export { default as Evented } from './lib/Evented';
 export * from './lib/Evented';