zero-keyword assignments

drop the `set` keyword, a la Python assignments

```
set src = img[data-testid=artwork-lightbox-image] -> xpath:@src
```

becomes simply:

```
src = img[data-testid=artwork-lightbox-image] -> xpath:@src
```

a detailed error message will be shown if attempting to assign to a reserved keyword. this requires reserving a block of keywords, even if they are currently unused