[web] Add protoc-gen-js dependency

Summary:
This is an addon to protoc that is necessary in order to run the script in the next diff. It's not usually packaged as an NPM package - context here.

The protoc in my local install has this addon already, which is why I didn't seem to need this change in my previous work in D7306. But the protoc in Nix doesn't seem to have this for some reason.

We could probably bring this in via Nix instead. I'm open to that if people prefer.

Test Plan:
Without this diff, the script in the following diff fails with the following error:

yarn run v1.22.19
$ ./scripts/codegen-identity-grpc.sh
protoc-gen-js: program not found or is not executable
Please specify a program using absolute path or make sure the program is available in your PATH system variable
--js_out: protoc-gen-js: Plugin failed with status code 1.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

After introducing the protoc-gen-js dependency, the script no longer errors.

Reviewers: varun, atul

Reviewed By: varun

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9490