New rule: Detect syntax errors in code examples

[jfmengels]

What the rule should do:

Detect syntax errors in code examples in module rules and in the README.

What problems does it solve:

If you don't use `elm-verify-examples, you have no way to know if your examples are correct. Checking that they are syntactically correct is a reasonable simple check.

I think it can be done relatively simple by using elm-syntax to parse the example.

Example of things the rule would report:

Any syntax error in code examples.

Maybe it would allow simple expressions? Behind a configuration maybe?

When (not) to enable this rule:

I am looking for:

• A good rule name. NoSyntaxErrorInExample?
• Someone to implement it (I can provide help and guidance)