Feature request: open file at position supports negative index

It would be nice if the line and column position supports the negative numbers and start counting from the end of the file.

Example:

We have a readme.md file filled with 20 lines.

edit readme.md:10 - opens the file at line 10
edit readme.md:-1 - opens the file at the last row
edit readme.md:-5 - opens the file at line 16

Start by tracing how the `edit readme.md:10` argument is parsed and how line and column positions are currently applied. Support negative positions relative to the end of the file, then verify that `-1` selects the last line and `-5` selects line 16 in a 20-line file.

Written by the indexing model from the issue text.