Suggestion: Cop for `sig` as a single or multi-line block

[andyw8]

Original context: Shopify/ruby-lsp-rails#131 (comment)

When a sig block has several parameters, it's usually more readable as a multi-line block, with one parameter per line. But when it only has one or two parameters, a single-line block is usually preferred.

Let's add a cop to auto-correct this.