[php-langspec][Issue #228] - isset variable list may include trialing ','

You can view the Issue on github:
https://github.com/php/php-langspec/issues/228

Opened By: zhujinxuan
Issue Description:
In php 7.3.0, isset is legal for the following code:
```
isset($a,);
```

However, the isset variable list is defined by 
```
isset-intrinsic:
   isset   (   variable-list   )

variable-list:
   variable
   variable-list   ,   variable
```
in `10-expression.md`. 

If we plan to make the spec consistent with current PHP, I can submit a PR to change it as 
```
isset-intrinsic:
   isset   (   isset-variable-list  ','? )

variable-list:
   variable
   variable-list   ,   variable
```