package.json: replace deprecated web-request with node-fetch@v2

`web-request` and its dependency `request` were deprecated
several years ago and are flagged with vulnerabilities.
Replace `web-request` with `node-fetch`.

web-request is currently used by the go release monitor in
goEnvironmentStatus and the gopls version check.

Fixes #2995

Change-Id: I180b3f805bf7462d7b0488f6111c69999e1ede7b
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/551115
Commit-Queue: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Peter Weinberger <[email protected]>

Author: hyangah