Enable DDC's Library Bundle module system by default

[Markzipan]

We're updating webdev to compile DDC apps using an updated Library Bundle module format by default, replacing the legacy AMD/Require module format. This new module format supports hot reload among other load-time optimizations.

For most standard DDC web apps, this change is completely transparent. However, your app might encounter issues if:

• You have a custom/generated index.html that contains hard-coded references to require.js or calls its methods (such as define or require).
• You have custom build scripts or use JS parsers that expect require.js or AMD-specific code patterns.
• Your app embeds/imports an AMD-compiled app from JS via require.js or from Flutter.

If you encounter build failures, script errors, or loader issues after updating, you can opt back into the legacy AMD module format with the --module-format flag:

webdev serve --module-format amd

webdev build --module-format amd