🏗 Pre-build JS files and extensions loaded by core runtime during gulp startup

[rsimha]

Background:

The default gulp task pre-builds the core runtime, and lazy builds all other JS files and extensions. This results in 404 errors for files that are sometimes loaded by the core runtime without a separate server request (#25024, #24226).

PR highlights:

• Pre-builds ww.js during default gulp startup
• Pre-builds amp-loader and amp-auto-lightbox during default gulp startup
• Waits for pre-build tasks to complete before ending default gulp task
• Fixes logging to indicate which extensions are being pre-built
• Server startup still happens before pre-build tasks are executed

Fixes #25024
Fixes #24226