Move command line tool to a new package named protobufjs-cli

[taylorcode]

I ran into issue #716 when trying to integrate pbjs and pbts into my build system. We must mirror our dependencies for reproducible builds.

@dcodeIO recommended that we convert cli/ into a new package. I see that the work here was started, but left in an incomplete state.

This PR attempts to finish decoupling protobufjs-cli from the parent package, so that it can be installed separately and so we can remove the behind the scenes npm install.

To test my changes I ran npm run test in the parent package. To test the binaries exposed by protobujs-cli package and verify that they do not depend on the parent package, I moved the /cli folder outside of the parent repo and invoked the binaries directly. I confirmed with the following commands:

pbjs --target static-module --wrap es6 protos/sample.proto

provided a proto with the content:

syntax = "proto3";

package sample_js_init_data;

message TestMessage {
    bool test_bool = 1;
    string test_string = 2;
}

It logs generated js.

When output to a file generated_js/sample.js, and running the following command:
pbts generated_js/sample.js

It logs the generated typings file content.

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Original PR with additional context:
#1234