Fix RuboCop::AST::NumericNode to return boolean

The documentation states that `sign?` returns boolean:

```ruby
# Checks whether this is literal has a sign.
#
# @example
#
#   +42
#
# @return [Boolean] whether this literal has a sign.
def sign?
  source.match(SIGN_REGEX)
end
```

but it does not.

Looks like the only usage [1] of `sign?` is this [2] module, so nothing should break.

[1] https://github.com/search?q=org%3Arubocop%20sign%3F&type=code
[2] https://github.com/rubocop/rubocop/blob/ddbb2a1bb65a29ac2d2d963196f7b00821779fd6/lib/rubocop/cop/style/method_call_with_args_parentheses/omit_parentheses.rb#L226

Author: viralpraxis

changelog/fix_numeric_node_sign_to_return_boolean.md

@@ -0,0 +1 @@
+* [#380](https://github.com/rubocop/rubocop-ast/pull/380): Fix `RuboCop::AST::NumericNode` to return boolean. ([@viralpraxis][])

lib/rubocop/ast/node/mixin/numeric_node.rb

@@ -15,7 +15,7 @@ module NumericNode
       #
       # @return [Boolean] whether this literal has a sign.
       def sign?
-        source.match(SIGN_REGEX)
+        source.match?(SIGN_REGEX)
       end
     end
   end