Write new formatting engine

PSSA2 currently only has an analyzer. It should also have a formatter.

In particular, the API should allow objects to implement an abstract class that doesn't emit diagnostics, but instead contributes edits to a formatting buffer.

Rules should be able to provide an implementation for this also by implementing an API that provides an instance of the required abstract class.

Requirements are:

• Run as many formatters over the script as it can at once
• When formatters want to overwrite the same extent, the largest wins
• Attempt to run until no more formatters have edits to make, up to some maximum number of runs
• Formatters should reuse the rule configuration system

首先阅读现有的分析器和规则配置系统，然后梳理 formatter API 如何与它们并存。完成的标准是支持 formatter 实例、具有“最大重叠优先”行为的并发编辑、重复运行直到达到上限，以及复用规则配置。