Support for numeric properties

Fixes object properties with a numeric name or a name with a leading number:

{
  "type": "object",
  "properties": {
    "1000": {
      "type": "integer"
    },
    "1000string": {
      "type": "string"
    }
  }
}

will result in the following interface (variables are prefixed as PHP doesn't allow variable names with a leading number):

public function get1000(): int;
public function set1000(int $numeric_property_1000): self;
public function get1000string(): string;
public function set1000string(string $numeric_property_1000string): self;