feat(textarea): add methods for moving the cursor to a given line

[meowgorithm]

We added the following methods:

• func(m *Model) SetCursorPosition(x, y int)
• func(m *Model) SetCursorRow(int)
• func(m *Model) SetCursorColumn(int)

SetCursor is now deprecated in favor of SetCursorColumn.

[bashbunni]

We should probably include some example usage on this

[dkotik]

Found this pull request, because I was going to make one just like it. Some additional ideas:

• textarea.CursorEnd should be renamed to textarea.CursorLineEnd
• textarea.CursorEnd should do the same as pressing Ctrl+End - jumping to end of input
• mirror the above two for CursorStart

[meowgorithm]

This one's a bit stale now—how do we feel about it? @lrstanley, wonder if you have any insight.

Can also just close it a well—easy enough to reimplement.