feat(build): add utils.deploy.env.add build plugin API

This changeset adds a new utility to build plugins:
`utils.deploy.env.add()`. This utility function allows build plugins to
register environment variables at build time that will be injected into
a specific deploy.

Userland build plugins cannot see or manipulate the deploy environment
variables registered by core or other userland plugins.

Deploy-specific environment variables are collected by the `build`
supervisor process and processed by the deploy step, which sends them to
Buildbot for processing. They are also returned by the programmatic `dev`
interface; we'll use these returned variables in the CLI, where we'll
e.g. set them on functions.

While I was in here I also converted a few files to TypeScript/generally
did my best to improve the types along the way.

Author: ndhoule

packages/build/src/core/dev.js

@@ -40,6 +40,6 @@ export const startDev = async (devCommand, flags = {}) => {
     const { severity, message, stack } = await handleBuildError(error, errorParams)
     const { success, severityCode } = getSeverity(severity)
 
-    return { success, severityCode, logs, error: { message, stack } }
+    return { success, severityCode, logs, error: { message, stack }, deployEnvVars: [] }
   }
 }