types: clearer types for presets

[SethFalco]

This defines clearer types for SVGO presets.

Regular plugins do not have the isPreset or plugins property defined. Whereas, presets always have both isPreset and plugins defined.

Before, we simply marked both as optional properties of a preset. The problem is that this prompted developers to do unnecessary checks, for example:

- if (plugin.isPreset && plugin.plugins) {
-   // Checking plugins shouldn't be neccesary. Presets always have a plugins array.
- }
+ if (plugin.isPreset) {
+   console.log(plugin.plugins); // We want to allow this.
+ }

This example is based on a community contribution we received:

• Updated plugin descriptions and added logic to set preset to the plugin description. #2174

We'll mark these properties as optional for normal plugins, but mandatory for presets. This also adds the benefit that if one checks BuiltinPluginOrPreset#isPreset and it returns true, then it will type-narrow the plugin to one of the presets.

For example:

const presetDefault = builtinPlugins.find((plugin) => plugin.name === 'preset-default')!;
if (!presetDefault.isPreset) {
  throw Error('Could not find preset-default.');
}

expectType<ReadonlyArray<BuiltinPlugin<string, Object>>>(presetDefault.plugins);
expectType<'preset-default'>(presetDefault.name);

It can now type-narrow the plugin to preset-default because we checked if it's a preset, and preset-default is the only preset available.

This also therefore introduces a rule where presets must be named preset-whatever. For example, preset-default, preset-html5, preset-svg2, etc.

Development Notes

What I really wanted:

• A single type so that documentation only had to be included once. ✅
• For the name, isPreset, and plugins property to type narrow in sync. ✅
• Use optional properties rather than hard-coding undefined. But, I had trouble achieving what I wanted here. ❌

I may revisit this later. Alternatively, feedback is always welcome, even after this PR is merged. 👍