Auto merge of #18134 - DropDemBits:source-change-sed, r=lnicola

internal: Extend SourceChangeBuilder to make make working with `SyntaxEditor`s easier

Part of #15710

Adds additional `SourceChangeBuilder` methods to make it easier to migrate assists to `SyntaxEditor`.

As `SyntaxEditor`s are composable before they're completed, each created `SyntaxEditor` can represent logical groups of changes (e.g. independently performing renames of uses in a file from inserting the new item). Once a group of changes is considered "done", `SourceChangeBuilder::add_file_edits` is used to submit a set of changes to be part of the source change.

`SyntaxAnnotation`s are used to indicate where snippets are attached to, and using `SyntaxAnnotation`s also means that we can attach snippets at any time, rather than being required to be after all edits.

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