🏗 Reorganize default gulp task to build extensions after launching server

[rsimha]

Today, the default gulp task builds the core runtime and all extensions before starting up its webserver. As the number of extensions has increased, the wait for server startup has become several minutes long.

This PR reorganizes the default gulp task as follows:

1. Build the core runtime and other default targets
2. Launch the webserver
3. Build extensions

With this PR, the wait for webserver startup goes down from a few minutes to ~30 seconds. The --watch and --extensions mechanisms remain unchanged.

Partial fix for #24141

[cramforce]

Thanks. I think this is a good first step and worth committing.

This is what I had in mind as an even better solution:

• block web server startup by nothing
• maybe eagerly build core runtime
• register a URL handler in the web server that does the following:
  • If the path points to a a file we could build
  • build the file now,
  • and when that is done respond with the result
  • unregister the handler and default to standard file serving.
  • in the luxury version, keep the path handler active and block responding when a watch-triggered build is active as well.
• That way we'd have quick startup and would just lazily build extensions as they are actually requested.