Adds dependencies for PostCSS and TypeScript with build
and develop scripts that use them in package.json.

Removes tsconfig parser option from eslintrc because it interferes
with a change in the tsconfig to exclude test files. We want eslint
to still lint test files. This is okay for now because none of our
lint rules depend on type information. If we add rules that require
type info we can create a separate tsconfig for eslint.

This change adds TypeScript but not esbuild. It is not
clear that we need minification so we will start with just
the TypeScript compiler for now. Looking at some of the sources
we currently minify with the closure compiler typical savings in
a file is less than 1kb. Once we have some TypeScript
code to compare minified and unminified performance with we
can consider adding esbuild or some other minifier.

For golang/go#43359
For golang/go#43360

Change-Id: I1a181952f546b0a0eb9b755128b2b83481e8ed92
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/280705
Reviewed-by: Julie Qiu <julie@golang.org>
Trust: Jamal Carvalho <jamal@golang.org>

This change moves pkgsite page styles to lib/css directory
and writes processed and minified css to static/css.

For golang/go#43360

Change-Id: I42d61f4a270e3cde5016c6063555eb037f35c302
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/281772
Trust: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jamal Carvalho <jamal@golang.org>