[release.theia] package.json: run npm dedupe

Duplicated package dependency was found and vsce package was unhappy.

For golang/vscode-go#1038

Change-Id: I10cb6ec1cf72bd163cdb5fde730c000b836506e7
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/278790


Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>