(#20002) Don't allow arrows instead of commas

For example selectors, function argument lists and case statements
allowed arrows (=>) instead of commas to separate items. This should not be
allowed according to docs.

Allowed for the following syntaxes for example:

```puppet
notice("foo" => "bar")
```

```puppet
$a = 3 ? {1=>2=>3=>4=>5=>6}
```