WIP for #31869

Initial support for a smart-string-copy-paste system. The general ideas here are:

1. if you copy code from VS, we can smartly determine what the original context was, to determine how to interpret characters. For example, if the user copies "" from inside a verbatim string (like @" goo "" bar "" baz "), then we can know this represents a single quote character.
2. when pasting code into a string literal we can use the above information to smartly determine to if we should escape the contents, and how to escape them. For example, if we see that we are pasting text whose value is a single quote character into a verbatim string, then we can double the quote to keep things legal. This allows copy/paste across different string types, while having the contents of the string stay the same, even if the escaping changes.
3. if pasting come in from an external source, we can't know the context of the text to know what values it actually has. However, we can still ensure that things are properly escaped to keep code parsable.

For all of the above, we ensure that any special work we do happens as a second edit transaction. THat way if the user ever doesn't want the contextual-pasting result, they can undo and get back to exactly the code that pastes normally.

This PR effectively adds the italicized items in this table:

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This is still a WIP as i work through all the combinations of source/destination text forms. In particular interpolated strings and raw strings are compelx in terms of how pasting has to work.