CHANGELOG.md: adopt "keep-a-changelog" style in CHANGELOG.md

And move extension/CHANGELOG.md to the repo root.

Fixes #3531

Change-Id: I0d81973a70d3b923e0d4d5e185a32192c6498401
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/613080
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>

Author: hyangah