Add option to touch only changed lines

It would be nice to have a mode to detect which lines you had touched in this editing session and only format those ranges.

We'd need a way to decide between this behavior and formatting the whole buffer. One option would be a new `:FormatChangedLines` and `:AutoFormatChangedLines` command that would work as variants of `:FormatCode` and `:AutoFormatBuffer`.

Start by reading the existing :FormatCode and :AutoFormatBuffer commands and how the plugin tracks buffer changes. Define how a session's changed line ranges are detected, then verify that the proposed :FormatChangedLines and :AutoFormatChangedLines variants format only those ranges while the existing commands retain whole-buffer behavior.

Written by the indexing model from the issue text.