Add new rule to SwiftLint for linting newlines before return statements

[svenmuennich]

We should add a rule to SwiftLint via PR that allows linting newlines before a return statement.

This cannot be linted using a regular expression with reasonable complexity and hence needs to be implemented in code. Once we implemented this we should send it as a PR to the main SwiftLint repo and see if they would like to add it. Hence we should think about configuration options, for which I propse the following:

• allowPrecedingComments: If set to true, a comment might precede a return statement without an empty line between them. In that case the comment must be preceded by a newline instead. true by default.

The rule should work similar to the one we already implemented for PHP.

The rule should be implemented using the AST provided by the new SyntaxRule.