EnC: Add UI that displays active changes as a diff

[tmat]

When updating multiple files in a solution while debugging it is not always easy to keep track of all changes being made. Especially as some files may be updated by various build tools (e.g. resources, source-generated files, etc).

It would be useful to add a command under the Hot Reload button that shows UI that displays the changes being made that haven't been applied yet. The UI could be similar to the diff UI we display after calculating Code Fix changes.

We can also display live changes in a dedicated tool window "Hot Reload: pending changes".