Main editor objects and it's methods definition

[ashumilova]

Such important objects of the Editor API should be covered along with their methods:

• TextEditor - an editor that is attached to a text document
• TextDocument - a source file, that has lines and knowledge about an underlying resource like a file
• TextLine - a line of source code
• Range - an ordered pair of two positions - start and end
• Position - a line and character position, such as the position of the cursor
• Selection - text selection in an editor

Subtasks:

• Define the editor api workflow needed to implement teletype plugin Define the editor api workflow needed to implement teletype plugin #9679
• Implement Editor API needed for a teletype plugin Implement Editor API needed for a teletype plugin #9680
• Implement a simple teletype plugin that would use the editor api Implement a simple teletype plugin that would use the editor api #9681