package.json: update webpack&webpack-cli dev dependencies

The original goal was to update 'ssri' to the latest version that includes
security fixes.

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We don't directly depend on ssri, so updated webpack, which has implicit
dependency with webpack-cli. So, updated webpack & webpack-cli using

npm install --save-dev webpack-cli@latest
npm install --save-dev webpack@latest

Now there is no dependency on ssri.
ssri was only on the dev dependency tree, so this shouldn't affect the extension.

Change-Id: Ieb5ecd1537cbb47757c66cc8a3ca4ca92f5ed7ca
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/303989
Trust: Hyang-Ah Hana Kim <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Suzy Mueller <[email protected]>