🏗 Lazily build JS during default gulp task

[rsimha]

#24152 added a way to lazily build extensions during the default gulp task. This PR goes a step further, and lazily builds all JS (except for the core runtime amp.js, which is eagerly built).

Usage:

gulp --lazy_build

Highlights:

• Removed all hardcoded compileJs calls in build-system/tasks/helpers.js
• Added several js target definitions to bundles.config.js
• Refactored JS building into a new function called doBuildJs()
• Added a new server module build-system/lazy-build.js to build JS and extensions
• Refactored performBuild() in build-system/tasks/build.js
• Refactored dist() in build-system/tasks/dist.js
• Assorted documentation / logging / clean up

Coming up:

• Refactor serve.js and server.js to eliminate the separate binary problem mentioned in 🏗 Lazily build extensions during the default gulp task #24152 (comment)
• Implement the "luxury" version mentioned in 🏗 Reorganize default gulp task to build extensions after launching server #24138 (review), where the server waits when a build is in progress

Partially addresses #24152 (review)
Partial fix for #24141
Follow up to #24138 and #24152

[cramforce]

Super cool!