$.utils.code.toSource is unnecessarily conservative about quoting property names

`$.utils.code.toSource({delete: 'foo'})` returns `{'delete': "foo"}`, but the single quotes around "delete" are not needed because although `delete` is a reserved word it is being used here as a property rather than variable name.

Similarly, `$.utils.code.toSource({4: 4})` returns `{'4': 4}` when `{4: 4}` is legal.

The `$.utils.code.quote` function is doing this but it perhaps needs to be forked into different versions for different contexts. (Note that you can also say `object.delete` but not `object.4`.)

Whatever changes are made here should be back to `server/code.js`.

Reference:

See https://262.ecma-international.org/5.1/#sec-11.1.5 (ES5.1) and/or https://tc39.es/ecma262/#sec-object-initializer (ES20xx) for the syntax of an _ObjectInitializer_; note that _PropertyName_ (ES5.1) / _LiteralPropertyName_ (ES20xx) can be any of **_IdentifierName_**, _StringLiteral_ or **_NumericLiteral_**, and that _IdentifierName_ is not the same as _Identifier_ (which does exclude _ReservedWord_).