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`.

首先阅读现有的 :FormatCode 和 :AutoFormatBuffer 命令，以及 plugin 如何跟踪 buffer 更改。定义如何检测一个 session 中已更改的行范围，然后验证所提议的 :FormatChangedLines 和 :AutoFormatChangedLines 变体只格式化这些范围，同时现有命令仍保持对整个 buffer 进行操作的行为。

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