Fix != operator definition

Problem: If a class declare the not-equal operator `!=`, the parsing
is wrong. For instance, the folowing program:

class A
  def !=(other); end
end

is parsed as if the method `!` was a defined with the body expression
syntax leading to the following tree:

(program [0, 0] - [3, 0]
  (class [0, 0] - [1, 20]
    name: (constant [0, 6] - [0, 7])
    body: (body_statement [1, 2] - [1, 16]
      (method [1, 2] - [1, 15]
        name: (operator [1, 6] - [1, 7])
        body: (parenthesized_statements [1, 8] - [1, 15]
          (identifier [1, 9] - [1, 14])))))
  (identifier [2, 0] - [2, 3]))

Solution: Declare `!=` as a possible operator definition.

Author: stac47

grammar.js

@@ -1025,7 +1025,7 @@ module.exports = grammar({
     )),
 
     operator: _ => choice(
-      '..', '|', '^', '&', '<=>', '==', '===', '=~', '>', '>=', '<', '<=', '+',
+        '..', '|', '^', '&', '<=>', '==', '===', '=~', '>', '>=', '<', '<=', '+', '!=',
       '-', '*', '/', '%', '!', '!~', '**', '<<', '>>', '~', '+@', '-@', '~@', '[]', '[]=', '`',
     ),