mark protobufjs as a dependency

[matiasgarcia]

Fixes #26

Fixes this error when importing datadog without the dependency:

Error: Cannot find module 'protobufjs/minimal'
Require stack:
- /usr/src/app/node_modules/@datadog/sketches-js/dist/ddsketch/proto/compiled.js
- /usr/src/app/node_modules/@datadog/sketches-js/dist/ddsketch/DDSketch.js
- /usr/src/app/node_modules/@datadog/sketches-js/dist/ddsketch/index.js
- /usr/src/app/node_modules/@datadog/sketches-js/dist/index.js
- /usr/src/app/node_modules/dd-trace/packages/dd-trace/src/histogram.js
- /usr/src/app/node_modules/dd-trace/packages/dd-trace/src/dogstatsd.js
- /usr/src/app/node_modules/dd-trace/packages/dd-trace/src/proxy.js
- /usr/src/app/node_modules/dd-trace/packages/dd-trace/src/index.js
- /usr/src/app/node_modules/dd-trace/packages/dd-trace/index.js
- /usr/src/app/node_modules/dd-trace/index.js
- /usr/src/app/dist/src/shared/datadog/tracer.js
- /usr/src/app/dist/src/main.js
    at Function.Module._resolveFilename (node:internal/modules/cjs/loader:1144:15)
    at Function.Module._load (node:internal/modules/cjs/loader:985:27)
    at Module.require (node:internal/modules/cjs/loader:1235:19)
    at Module.Hook.Module.require (/usr/src/app/node_modules/dd-trace/packages/dd-trace/src/ritm.js:64:27)
    at require (node:internal/modules/helpers:176:18)
    at Object.<anonymous> (/usr/src/app/node_modules/@datadog/sketches-js/dist/ddsketch/proto/compiled.js:3:17)
    at Module._compile (node:internal/modules/cjs/loader:1376:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1435:10)
    at Module.load (node:internal/modules/cjs/loader:1207:32)
    at Function.Module._load (node:internal/modules/cjs/loader:1023:12)
Waiting for the debugger to disconnect...

[orta]

This PR shouldn't be merged, and really should be closed, Protobufjs is intentionally not included because it is not a feature which everyone will use. It mentions this right at the start of the readme:

When using Protobuf serialization, the protobufjs module must also be installed manually:

And a bunch of work was done to make sure it doesn't leak into everyone's code #19 - I've been using the library just fine without the protobufjs module included, so it may be specific to your build system?