Improve <embed-code> tag parsing

[Vladyslav-Kuksiuk]

This PR:

• Fixes multiline attributes parsing for the <embed-code> tag.
  Previously, the multiline tag without additional spaces could not be parsed correctly, now it works:

  <embed-code
  file="org/example/Hello.java"
  start="public class Hello"
  end="System.out.println" />
  ```java
  ```
  

• Improves related error messages, for example:
  
  instead of:

  index out of range [-1]
  

• Improves work with file/folder paths.

[dmytro-kashcheiev]

@Vladyslav-Kuksiuk LGTM