fix: support babel config file names other than .babelrc

I've added find-up as a dependency for finding the babel config file.
The babel config files to look for have been taken from @babel/core's sourcecode
Finally, as babel-loader itself will by default not look outside of the cwd
for the config file, we don't want to have false positives for any babel config
files that exist outside of the cwd

Closes netlify#200, netlify#188

Author: Robin-Hoodie

lib/build.js

@@ -3,6 +3,27 @@ var path = require('path');
 var conf = require('./config');
 var webpack = require('webpack');
 var merge = require('webpack-merge');
+const findUp = require('find-up');
+
+/*
+ * Possible babel files were taken from
+ * https://github.com/babel/babel/blob/master/packages/babel-core/src/config/files/configuration.js#L24
+ */
+
+const BABEL_ROOT_CONFIG_FILENAMES = [
+  'babel.config.js',
+  'babel.config.cjs',
+  'babel.config.mjs',
+  'babel.config.json'
+];
+
+const BABEL_RELATIVE_CONFIG_FILENAMES = [
+  '.babelrc',
+  '.babelrc.js',
+  '.babelrc.cjs',
+  '.babelrc.mjs',
+  '.babelrc.json'
+];
 
 const testFilePattern = '\\.(test|spec)\\.?';
 
@@ -15,25 +36,22 @@ function getBabelTarget(envConfig) {
   return unknown ? '8.15.0' : current.replace(/^nodejs/, '');
 }
 
-function haveBabelrc(functionsDir) {
-  const cwd = process.cwd();
-
-  return (
-    fs.existsSync(path.join(cwd, '.babelrc')) ||
-    functionsDir.split('/').some((dir) => {
-      const indexOf = functionsDir.indexOf(dir);
-      const dirToSearch = functionsDir.substr(0, indexOf);
-
-      return fs.existsSync(path.join(cwd, dirToSearch, '.babelrc'));
-    })
-  );
+function existsBabelConfig(functionsDir) {
+  const babelConfigFile = findUp.sync([...BABEL_ROOT_CONFIG_FILENAMES, ...BABEL_RELATIVE_CONFIG_FILENAMES], {
+    cwd: path.join(process.cwd(), functionsDir)
+  });
+  if (babelConfigFile) {
+    // Ensure babel config files outside of working dir are not accounted for
+    return babelConfigFile.includes(process.cwd());
+  }
+  return false;
 }
 
 function webpackConfig(dir, { userWebpackConfig, useBabelrc } = {}) {
   var config = conf.load();
   var envConfig = conf.loadContext(config).environment;
   var babelOpts = { cacheDirectory: true };
-  if (!haveBabelrc(dir)) {
+  if (!existsBabelConfig(dir)) {
     babelOpts.presets = [
       [
         require.resolve('@babel/preset-env'),