[FR] support language services for functions inside of an environment

I often use "environment" as the namespace of R, so that I can manage the name conflicts in a large R project. Also, it's much easier to find the source of the function, with such "namespace".

However, using this pattern in VSCode R is not very convenient, as it can't provide any parameter info when calling functions inside of an environment (or a list).

It's probably difficult to provide "static" analysis for environment functions. But I think, at least we should use the current session info, as RStudio IDE does.

Thanks.

normal_fun <- function(a, b) {
  a + b
}

env <- new.env()
env$fun <- function(a, b) {
  a + b
}

# move curser on normal_fun, vscode tips the arguments; type a inside the bracket, it hints `a` is an argument
normal_fun()

# in contrast, a function inside an environment loses such ability
env$fun()

Start by tracing the language-server entry point that provides parameter information, using the issue's normal_fun and env$fun examples as the comparison case. Determine how current-session information could identify functions stored in environments or lists, and consider the work complete when calls such as env$fun() receive the same parameter hints as normal_fun().