Fix method name validation edge-case

It turns out that `:4_to_5=` is not a valid symbol but `"4_to_5=".to_sym`
does not quote it and represents it as `:4_to_5=`, which throws off our
whole method name validation logic.

The fix is to check validity of the name after stripping trailing `=`
as well. That leaves `==` as an edge-case, that we handle separately.

Author: paracycle

lib/tapioca/helpers/rbi_helper.rb

@@ -93,7 +93,10 @@ def self.serialize_type_variable(type, variance, fixed, upper, lower)
 
     sig { params(name: String).returns(T::Boolean) }
     def valid_method_name?(name)
-      !name.to_sym.inspect.start_with?(':"', ":@", ":$")
+      name == "==" || !(
+        name.to_sym.inspect.start_with?(':"', ":@", ":$") ||
+        name.delete_suffix("=").to_sym.inspect.start_with?(':"', ":@", ":$")
+      )
     end
 
     sig { params(name: String).returns(T::Boolean) }