Implement preset-default plugin (#1513)

I saw complaints about `extendDefaultPlugins` api

- it cannot be used when svgo is installed globally
- it requires svgo to be installed when using svgo-loader or svgo-jsx
- it prevents using serializable config formats like json

In this diff I introduced the new plugin which is a bundle of all
default plugins.

```js
module.exports = {
  plugins: [
    'preset_default',
    // or
    {
      name: 'preset_default',
      floatPrecision: 4,
      overrides: {
        convertPathData: {
          applyTransforms: false
        }
      }
    }
  ]
}
```

plugins/mergePaths.js

@@ -4,6 +4,7 @@ const { detachNodeFromParent } = require('../lib/xast.js');
 const { collectStylesheet, computeStyle } = require('../lib/style.js');
 const { path2js, js2path, intersects } = require('./_path.js');
 
+exports.name = 'mergePaths';
 exports.type = 'visitor';
 exports.active = true;
 exports.description = 'merges multiple paths in one if possible';