cpp: Add injection for raw string literals (#13726)

Release Notes:

- N/A

Before: 

![Screenshot_20240701_231801](https://github.com/zed-industries/zed/assets/66138117/d0df7819-09e7-4a3b-949d-78e04ff63b23)

After:

![Screenshot_20240702_162856](https://github.com/zed-industries/zed/assets/66138117/943136e1-3b15-482e-bf45-2571cd212eaf)

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>