Add Visual Indent Option in `no_code` Layout

When using `no_code` layout, pre-text and post-text are often provided.

In order to make the indentation of the solution appear visually coherent, it would be convenient to provide a `visual-indent="n"` attribute on the element, that simply nudges the tray `n` character widths to the right relative to pretext.

### Visual Example

We want to emulate the experience of writing inline with the context, like so:
```ruby
# this is our parsons problem

describe GiftCard do # pre-text
context "#withdraw" do #
+--------------+
>>>>>>>>| solution box | # box with visual-indent="8"
+--------------+
end # post-text
end #
```

**not**
```ruby
describe GiftCard do
context "#withdraw" do
+--------------+
| solution box | # poorly-aligned, confusing, lame
+--------------+
end
end
```