fix: add missing peer dependency html-webpack-plugin

[merceyz]

What's the problem this PR addresses?

critters-webpack-plugin tries to require html-webpack-plugin (

critters/packages/critters-webpack-plugin/src/index.js

Lines 71 to 75 in 5e32531

	try {
	var htmlPluginHooks = require('html-webpack-plugin').getHooks(
	compilation
	);
	} catch (err) {}

) but doesn't declare it as a dependency which means under strict dependency environments or if hoisting isn't in its favour it wont get access to it.
https://yarnpkg.com/advanced/rulebook/#packages-should-only-ever-require-what-they-formally-list-in-their-dependencies

Ref preactjs/preact-cli#1608 (comment) (cc @rschristian)

How did you fix it?

Based on how critters-webpack-plugin is trying to use html-webpack-plugin I added it as an optional peer dependency.