Lexical variables in the outermost lexical scope don’t get mangled

Lexical variables in the outermost lexical scope don’t get mangled, i.e. they appear to be incorrectly treated as global variables.

**To Reproduce**

Minimal code to reproduce the bug:

```js
// foo.mjs
const foo = 42;
const bar = 64;
```

```sh
$ minify foo.mjs
```

**Actual Output**

```js
const foo=42,bar=64;
```

**Expected Output**

```js
const a=42,b=64;
```

I’m using the `babel-minify` CLI, v0.4.3, with no additional configuration.

As a workaround, I have to wrap my source code in a block: `{ … }`. That way, babel-plugin-minify-mangle-names seems to kick in correctly.

Start by reproducing the issue with the babel-minify CLI and the foo.mjs example, then inspect babel-plugin-minify-mangle-names and how it handles the outermost lexical scope. Done means the top-level variables are mangled to output such as const a=42,b=64, without requiring an extra block.

Written by the indexing model from the issue text.