feat: expose builtin plugins

[SethFalco]

Exposes a new property called builtinPlugins. This is intended to cover use cases where clients imported/required the list of available plugins or the default preset.

If you import/require the array of built-in plugins, or a single plugin during runtime, this is now a top-level exported member instead:

// builtin.mjs - get an array of all built-in plugins
- import { builtin } from 'svgo/lib/builtin';
+ import { builtinPlugins } from 'svgo'

// plugin.mjs - get a single plugin
- import presetDefault from 'svgo/plugins/preset-default';
+ import { builtinPlugins } from 'svgo';
+ const prefixDefault = builtinPlugins.find(plugin => plugin.name === 'preset-default');

// plugin-map.mjs - get all plugins as a map using the plugin name as a key
import { builtinPlugins } from 'svgo';
const pluginMap = builtinPlugins.reduce((acc, val) => acc.set(val.name, val), new Map());
const prefixDefault = pluginMap.get('preset-default');

This also introduces the proposal from:

• chore: add optional schema prop to plugins #1943 (comment)

Plugins may now export two more properties, isPreset and plugins, both only defined for presets such as preset-default. This is aimed at client developers rather than end-users, and allows them to check if a plugin is a preset and get the plugins in a preset in the order they they are invoked.