Simplify single-line embedding

[Vladyslav-Kuksiuk]

This PR improves the single-line embedding, by adding the line attribute to the <embed-code> instruction. Instead of providing both similar start and end it is now possible to provide one line attribute.

For example:

<embed-code
  file="$test/issue19.kts"
  line="val validationVersion"
/>
```
val validationVersion by extra("2.0.0-SNAPSHOT.419")
```

Note: The line attribute cannot be combined with start, end, or fragment.

Resolves SpineEventEngine/embed-code-jekyll#19.

[dmytro-kashcheiev]

@Vladyslav-Kuksiuk LGTM