Issues with generating source maps

[piotr-cz]

1. The --sourcemap option is available only for the wmr start command (defined here) - but doesn't trigger source maps generation.

2. When using wmr build --sourcemap (undocumented) - source maps are generated, but following warning shows up in console:

   Sourcemap is likely to be incorrect: a plugin (wmr-styles) was used to transform files, but didn't generate a sourcemap for the transformation. Consult the plugin documentation for help
   

   This is probably caused by missing map property: in the transform method

   wmr/packages/wmr/src/plugins/wmr/styles-plugin.js

   Lines 175 to 179 in e8b5880

   	return {
   	code,
   	moduleSideEffects: true,
   	syntheticNamedExports: true
   	};

[developit]

Hiya! Good catch. We don't actually support sourcemaps during dev because there is so little transformation happening. We could in prod though, that plugin should just be returning { map: null }.

[piotr-cz]

@developit Thank you for tips! 👍