Update to partial attribute handling

Now use a much more natural syntax - similar to text interpolation:

- `value="some constant value"` >> `partial.value = "some constant value`
- `value="$name"` >> `partial.value = ""+name`;
- `value="$name@gmail.com"` >> `partial.value = ""+name+"@gmail.com";`
- `value="${name.toUpperCase()}"` >> `partial.value = ""+name.toUpperCase();`

Unit tests for a few more cases added.

Also, all "dtx-*" attributes on a partial call are now reserved for internal use,
in case we want more features similar to `dtx-name` in future.