This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

As the number of tree shaking options is growing, this will provide the user to additionally choose from three presets: safest, smallest and recommended.

The differences are that while smallest uses the most aggressive setting everywhere and safest uses the most conservative, recommended will be conservative on most settings but may swallow some syntactic errors. At the moment, it will swallow errors from accessing unknown global variables, but in the future this may extend to accessing const/let variables before they are initialized.

You can use if like this:

export default {
  treeshake: 'safest',
  // ...
} 

There is also an object form that allows using a preset while overriding some values:

export default {
  treeshake: {
    preset: 'smallest',
    propertyReadSideEffects: true
  },
  // ...
} 

From the CLI, you use it like this

rollup -c --treeshake smallest

Using a preset from the CLI will of course override any values in the config file.