Update @babel/helper-wrap-function templates (#6984)

This commit introduces 4 changes:

1) Function declarations are wrapped using function declarations.
   This has two advantages:
    - We can rely on native hoisting, instead of using _blockHoist
    - The function isn't wrapped until it is called. This avoids
      problems where `regeneratorRuntime.wrap` was called before
      that `babel-polyfill` was imported.

   Example:
     function fn() {}
     // becomes
     function fn() { return _fn.apply(this, arguments); }
     function _fn() {
       _fn = _wrapper(/* Original function ... */);
       return _fn.apply(this, arguments);
     }

2) Use a single template for both named and anonymous function
   expressions. They already had the same behavior, but the one
   used for named functions was a bit longer.

3) Use normal functions instead of arrow functions to wrap
   function expressions.

4) Generate a name based on the original one for wrapped
   functions (e.g. `foo` becomes `_foo` instead of `_ref`).

packages/babel-plugin-proposal-function-sent/test/fixtures/function-sent/basic/expected.js

@@ -1,13 +1,14 @@
-let gen = (() => {
-  var _ref = _skipFirstGeneratorNext(function* () {
+function _skipFirstGeneratorNext(fn) { return function () { var it = fn.apply(this, arguments); it.next(); return it; }; }
+
+function gen() {
+  return _gen.apply(this, arguments);
+}
+
+function _gen() {
+  _gen = _skipFirstGeneratorNext(function* () {
     let _functionSent = yield;
 
     let sent = _functionSent;
   });
-
-  return function gen() {
-    return _ref.apply(this, arguments);
-  };
-})();
-
-function _skipFirstGeneratorNext(fn) { return function () { var it = fn.apply(this, arguments); it.next(); return it; }; }
+  return _gen.apply(this, arguments);
+}