[@kbn/babel-register] ensure that source maps are included inline (#1…

…48407)

In #146212 the default
`sourceMaps` config for babel register changed from `both` to `true`,
which creates the source maps but does not embed them in the compiled
output so that dev-tools are able to inspect them even though they are
not on the filesystem.

cc @vitaliidm

packages/kbn-babel-transform/options.js

@@ -31,7 +31,7 @@ function getBabelOptions(path, config = {}) {
     ],
     cwd,
     babelrc: false,
-    sourceMaps: !config.disableSourceMaps,
+    sourceMaps: config.disableSourceMaps ? false : 'both',
     ast: false,
   };
 }