added `ConvertStringConcatenationToStringInterpolation` to convert string concatenation to string interpolation:
- the string concatenation must contain at least one string literal
- the number of pound symbols in the resulting string interpolation is determined by the highest number of pound symbols among all string literals in the string concatenation
- multiline string literals are not supported
registered in `SyntaxCodeActions.allSyntaxCodeActions`
registered in Sources/SourceKitLSP/CMakeLists
created a test in `CodeActionTests`