chore(babel): add deprecation/migration warnings for the runtime (#534)

* docs(babel): Add warning to tell about configuration name change runtimeHelpers -> babelHelpers

This was figured out after hours of poking.

* docs(babel): Tweak deprecation warnings about runtimeHelpers & externalHelpers options

Co-authored-by: Mateusz Burzyński <mateuszburzynski@gmail.com>

packages/babel/src/index.js

@@ -29,8 +29,26 @@ const unpackOptions = ({
   };
 };
 
+const warnAboutDeprecatedHelpersOption = ({ deprecatedOption, suggestion }) => {
+  // eslint-disable-next-line no-console
+  console.warn(
+    `\`${deprecatedOption}\` has been removed in favor a \`babelHelpers\` option. Try changing your configuration to \`${suggestion}\`. ` +
+      `Refer to the documentation to learn more: https://github.com/rollup/plugins/tree/master/packages/babel#babelhelpers`
+  );
+}
+
 const unpackInputPluginOptions = ({ skipPreflightCheck = false, ...rest }, rollupVersion) => {
-  if (!rest.babelHelpers) {
+  if ('runtimeHelpers' in rest) {
+    warnAboutDeprecatedHelpersOption({
+      deprecatedOption: 'runtimeHelpers',
+      suggestion: `babelHelpers: 'runtime'`
+    });
+  } else if ('externalHelpers' in rest) {
+    warnAboutDeprecatedHelpersOption({
+      deprecatedOption: 'externalHelpers',
+      suggestion: `babelHelpers: 'external'`
+    });
+  } else if (!rest.babelHelpers) {
     // eslint-disable-next-line no-console
     console.warn(
       "babelHelpers: 'bundled' option was used by default. It is recommended to configure this option explicitly, read more here: " +