…dit` (#10855)

Fixes #10842

The formatting self-nerd-sniping continues.

The formatting engine was written to use the LSP `TextEdit` class, which
makes some sense, but also uses Roslyn APIs like `SourceText` a lot,
which uses the `TextChange` struct instead. This meant lots of code to
convert to and from the two types. Changing the whole formatting engine
over to `TextChange`, and using more `TextSpan`, `LinePositionSpan` etc.
removes a lot of this code. It also makes a lot more sense in cohosting,
to boot.

I wouldn't claim that I've gone through and improved the perf of the
formatting engine, but rather I've use the changes to lead me to things
that need fixing. ie, I started out moving from `TextEdit[]` to
`ImmutableArray<TextChange>`, and this let me to places where pooled
array builders could be used, and places where `Range` and `Position`
were used which didn't make much sense, and then the constructor for
`LinePosition` threw at one point because it turns out we were only
using the `Line` property from the `Position` that used to be used, and
so never validated the characters, so that API moved to `int`, etc.

TL;DR the commits tell the story, and there could well be something I
missed, if it never came across my plate for another reason.