🏗✨ Enable lazy-build and watch modes for minified code

[rsimha]

Background:

With #27426 and #27447, the pre-closure babel transforms run by gulp dist were refactored from all-at-once to on-demand. This paves the way for lazy-build and watch modes for minified code.

PR Highlights:

• Introduces watch mode for minified code
• Standardizes minified and unminified usage via —watch
• Introduces lazy-build mode for minified code
• Reduces minified startup from ~7m (gulp dist) to <1m (gulp --compiled)
• Standardizes error handling during compilation and cleans up error logging
• Cleans up internal flags and options
• Updates stale documentation

Newly added commands:

# Unminified watch build
gulp build --watch # Replaces gulp watch

# Minified watch build
gulp dist --watch

# Minified watch build for just the core runtime
gulp dist --watch --core_runtime_only

# Minified watch build for some extensions
gulp dist --watch --extensions=amp-foo,amp-bar

# Minified watch build for extensions used by an example file
gulp dist --watch --extensions_from=examples/foo.amp.html 

# Minified lazy + watch build
gulp --compiled

# Minified lazy + watch build (pre-build some extensions)
gulp --compiled --extensions=amp-foo,amp-bar

# Minified lazy + watch build (pre-build extensions used by an example file)
gulp --compiled --extensions_from=examples/foo.amp.html 

Examples:

gulp --compiled

gulp dist --watch --core_runtime_only

Follow up to #27426 and #27447

[cramforce]

Nice!
CC @kristoferbaxter as this is very relevant for performance debugging. We should also make sure more people know about pseudo optimized code.

[samouri]

This change is awesome ⭐️