extension/LICENSE: copy the latest LICENSE file from release

The release branch kept the expanded LICENSE file list that
includes all the production dependency's license files.
Place them permanently in the extension directory.
The subsequent change will add a test that checks this
file is consistent with the dependency info in package.json.

Change-Id: I663779e3d08a01cf552693522946c3ec3a047939
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/610256
kokoro-CI: kokoro <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>
Commit-Queue: Hyang-Ah Hana Kim <[email protected]>

Author: hyangah