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 <[email protected]>
kokoro-CI: kokoro <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>

Author: hyangah