Updates to R syntax with recent language features #4190

Related: #4040

The R language definition (components/prism-r.js in v1 / src/languages/r.ts in v2) has not been updated for R 4.0 / R 4.1 syntax additions:

1. Raw string literals (?Quotes):
   • Currently tokenized as an identifier r followed by a regular string (which breaks when the raw string contains unescaped quotes or backslashes like r"(C:\path\to\"file")")
2. Native pipe |> (?pipeOp):
   • Currently split into | + > (both operators).
   • Relatedly, the native placeholder _ is not supported
3. Shorthand lambda \(...) (?function)
   • \ in \(x) x + 1 is currently unhighlighted.
4. Variadic ..N should be reserved just as ... is

Prism also lacks support for some very common core language features for R:

1. Internal namespace operator (:::) and walrus assignment (:=):
   • operator currently matches ::? (leaving the third : in pkg:::fn separate) and does not match := (used heavily in {data.table} and {rlang}).
2. {roxygen2} documentation comments (#' @param ...):
   • #' doc comments and @tag directives (@param, @return, @export, etc.) are extremely common in R packages.
3. Function calls:
   • prism-r currently defines no function token rule, unlike most other Prism grammars.
4. S4 slot extraction operator @ is not recognized (?methods::slot)
5. Multi-line string support as highlighted in Multiline R strings are not correctly highlighted #4040

Here is an example R snippet demonstrating some of the missing features:

#' @param x_val Input vector
#' @export
`my-fn` <- \(x_val = 1L, ...) {
  raw_str <- r"--(c:\path\to\file)--"
  pkg::pub_fn(x_val, ..1) |>
    lm(y ~ x, data = _) |>
    _$coefficients
  dt[, col := obj@slot]
}

I can propose some PRs to fix this, but I am not sure where best to direct the efforts given the v1/v2 split. Is v2 stable enough to start accepting patches to "more peripheral" grammars like R?