[JavaScript] Improve arrow function fallback behavior

[Thom1729]

See babel/babel-sublime#348 (comment).

Example:

({
    a = {},
//    ^ keyword.operator.assignment
//      ^^ punctuation.section.block
//        ^ punctuation.separator.comma - keyword.operator.comma
    b,
//   ^ punctuation.separator.comma - keyword.operator.comma
}) => null;
// ^^ storage.type.function.arrow

The arrow function parameter list will be misparsed as a parenthesized expression and the destructured parameters as an object literal. (See sublimehq/sublime_text#2241.) To accommodate this, we handle = signs inside object literals to provide the best possible fallback behavior. Without this, some parameter default values can break highlighting for the rest of the object literal.