feat(ability): split Ability into PureAbility (base class) and `A…

…bility` (preconfigured PureAbility)

This should make migration easier and at the same time open more possibilities to configure `Ability` and improves tree shaking support

Relates to #249 BREAKING CHANGE: `AbilityBuilder.define` was extracted into `defineAbility` function and `AbilityBuilder.extract` was removed

**Before**

```js
import { AbilityBuilder, Ability } from '@casl/ability'

const ability = AbilityBuilder.define((can) => {
  can('read', 'Post')
})

// the same with extract
const { can, rules } = AbilityBuilder.extract()

can('read', 'Post')

const ability = new Ability(rules)
```

**After**

```js
import { defineAbility, Ability, AbilityBuilder } from '@casl/ability'

const ability = defineAbility((can) => {
  can('read', 'Post')
})

// the same with constructor
const { can, rules } = new AbilityBuilder()

can('read', 'Post')

const ability = new Ability(rules)
```

package.json

@@ -35,7 +35,7 @@
     "rollup": "^1.1.2",
     "rollup-plugin-babel": "^4.3.2",
     "rollup-plugin-node-resolve": "^5.0.0",
-    "rollup-plugin-uglify": "^6.0.2",
+    "rollup-plugin-terser": "^5.2.0",
     "semantic-release": "15.9.17",
     "tslib": "^1.9.3",
     "typescript": "~3.7.0"

packages/casl-ability/package.json

@@ -17,7 +17,7 @@
     "access": "public"
   },
   "scripts": {
-    "build.core": "rollup -c ../../tools/rollup.config.js -i src/index.ts -g sift:sift -n casl",
+    "build.core": "rollup -c ../../tools/rollup.config.js -i src/index.ts -g sift:sift,sift/operations:sift.operations -n casl",
     "build.extra": "rollup -c ../../tools/rollup.config.js -i src/extra.ts -n casl.extra",
     "build.types": "rm -rf ./dist/types/* && tsc",
     "build": "npm run build.core && npm run build.extra && npm run build.types",