Node example on README doesn't work

Tried the example provide to test locally with Node and the code provided returns error:

```
linter.MetaCharsetIsFirst(context).then(console.log);
^

TypeError: linter.MetaCharsetIsFirst is not a function
at Object. (/Users/ibrahim.souza/Projetos/webstories-linter/linter.js:12:8)
at Module._compile (internal/modules/cjs/loader.js:1138:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10)
at Module.load (internal/modules/cjs/loader.js:986:32)
at Function.Module._load (internal/modules/cjs/loader.js:879:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
at internal/main/run_main_module.js:17:47
```

Only through local build I got the result expected to evaluate our code. Since we are a small team, if this worked as documented would be better for our team non-technical run the commands without the hassle of build this repo and navigate through folders.

Node version: **12.18.0**
Npm version: **6.14.4**
System: **Mac OSX 10.15.5**
@ampproject/toolbox-linter: **2.5.12**

Start with the Node example in the README and compare its exported API with the @ampproject/toolbox-linter package at version 2.5.12; reproduce with Node 12.18.0. Done means the documented command runs without a local build and returns the expected lint result.

Written by the indexing model from the issue text.