Separate formatting of text edits made to a document from formatting all or part of a document

[davidwengier]

There is a quirk in how our on type formatting engine is used.

Before a recent change our on type formatting worked by having the on type formatting endpoint call Roslyn to do on type formatting, and then passing those edits into our formatting engine, to run through all of the formatting passes.

After the change, things are more consisten: the call to Roslyn is in the CSharpOnTypeFormattingPass itself, because that is where logically it should belong (the call to Roslyn for whole document formatting is in CSharpFormattingPass and the call to WebTools for Html formatting is in HtmlFormattingPass). Unfortunately there are other things (snippets, completion etc.) which construct a set of edits and then call our formatting engine pretending to be on type formatting.

The formatting engine has an allowance for this, to not break those scenarios, and can either format edits passed in to it, or it can go and ask Roslyn for some edits. Obviously this is crazy, and should be separated out.

Originally posted by @davidwengier in #6064 (comment)

[davidwengier]

This issue only applies once #6064 is complete. I've marked it help wanted because test coverage is good, and this should be a pure refactoring, so shouldn't need a lot of specific domain knowledge.