Implement prettier.getFileInfo() method and --file-info CLI option (#4341)

* Implement prettier.getFileInfo() method and --file-info CLI option

* Add empty line between functions in index.js

* Support --plugin-search-dirs / pluginSearchDirs() in --file-info / getFileInfo()

* Address review comments by @ikatyang

tests_integration/plugins/automatic/node_modules/@prettier/plugin-foo/index.js

@@ -6,7 +6,8 @@ module.exports = {
   languages: [
     {
       name: "foo",
-      parsers: ["foo"]
+      parsers: ["foo"],
+      extensions: [".foo"]
     }
   ],
   parsers: {