Add initial work on composite-checkout package (#36720)

This will be a new package that can be used to replace the checkout process in Calypso, CrowdSignal, Jetpack, and Akismet.

This is not used anywhere yet, but by merging into master it will become easier to iterate on the work.

package.json

@@ -189,6 +189,7 @@
 		"source-map-support": "0.5.12",
 		"store": "2.0.12",
 		"striptags": "2.2.1",
+		"styled-components": "4.4.0",
 		"superagent": "3.8.3",
 		"svg4everybody": "2.1.9",
 		"textarea-caret": "3.1.0",
@@ -286,7 +287,8 @@
 		"validate-fallback-es5": "npx eslint --env=es5 --no-eslintrc --no-ignore ./public/fallback/*.js",
 		"postshrinkwrap": "node -e \"fs.utimesSync( './node_modules', new Date(), new Date() );\"",
 		"prewhybundled": "cross-env-shell CALYPSO_ENV=production EMIT_STATS=withreasons NODE_ARGS=--max_old_space_size=8192 npm run -s build-client",
-		"whybundled": "whybundled stats.json"
+		"whybundled": "whybundled stats.json",
+		"composite-checkout-demo": "webpack-dev-server --config ./packages/composite-checkout/webpack.config.demo.js --mode development"
 	},
 	"devDependencies": {
 		"@automattic/babel-plugin-i18n-calypso": "file:./packages/babel-plugin-i18n-calypso",
@@ -295,6 +297,7 @@
 		"@automattic/calypso-color-schemes": "file:./packages/calypso-color-schemes",
 		"@automattic/webpack-extensive-lodash-replacement-plugin": "file:./packages/webpack-extensive-lodash-replacement-plugin",
 		"@automattic/webpack-inline-constant-exports-plugin": "file:./packages/webpack-inline-constant-exports-plugin",
+		"@automattic/composite-checkout": "file:./packages/composite-checkout",
 		"@types/classnames": "2.2.9",
 		"@types/debug": "4.1.4",
 		"@types/lodash": "4.14.144",
@@ -361,6 +364,9 @@
 		"supertest": "3.4.2",
 		"svgstore-cli": "1.3.1",
 		"typescript": "3.6.4",
+		"webpack": "4.41.2",
+		"webpack-cli": "3.3.9",
+		"webpack-dev-server": "3.8.2",
 		"webpack-hot-middleware": "2.25.0",
 		"webpack-node-externals": "1.7.2",
 		"whybundled": "1.4.2"