Types and examples omit 'premajor', 'preminor' and 'prepatch'

[narkowicz]

The TypeScript literal return type semverDiff.Result and examples in README, JSDoc omit the following possible values that can be returned internally by node-semver:

• 'premajor'
• 'preminor'
• 'prepatch'

semverDiff('1.0.0-foo', '2.0.0');
//=> 'premajor'
semverDiff('1.0.0-foo', '1.1.0');
//=> 'preminor'
semverDiff('1.0.0-foo', '1.0.1');
//=> 'prepatch'

As in npm/node-semver#83 (comment)