[Feature]: Better log message when sass/less plugin is not installed

[Timeless0911]

What problem does this feature solve?

Rslib supports using sass/less as css precessor, we should log some user-friendly error message if user do not install and configure@rsbuild/plugin-less / @rsbuild/plugin-sass.

What does the proposed API look like?

[chenjiahan]

Rsbuild already provides logs for this case:

[Timeless0911]

My bad, didn't notice it at once😂