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 theintern#1018

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

devPackages/eslint-config/index.js

@@ -8,6 +8,7 @@ module.exports = {
     "project": [
       "./packages/*/tsconfig.json",
       "./packages/*/tests/tsconfig.json",
+      "./packages/core/tests/examples/tsconfig.json",
       "./devPackages/*/tsconfig.json"
     ]
   },

packages/common/src/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';