Add support for enhancements for pipe placeholder `_` added in R language 4.3

Since R language 4.3, the pipe placeholder _ can also be used in the rhs of a forward pipe |> expression as the first argument (head) in an extraction call.

• Add support to R parser
• Take it into account in SourceAnalyzer
• Adapt error detection and reporting
• Check support for syntax highlighting
• Add support for expr index computation