cmd/compile: add node collapse/expand to html ast output

cmd/compile: add node collapse/expand to html ast output

AI-generated code, plus a lot of reviewing and tweaking
of arrows and fonts.

This adds subtree collapse/expand triangles.

Change-Id: I2dd322abdf7ef956b1435946d79f864a6150f976
Reviewed-on: https://go-review.googlesource.com/c/go/+/740481
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>