Emit R6Class methods as workspace symbols

[lionel-]

Addresses posit-dev/positron#6549

• Add a bunch of tests for the workspace indexers which were not tested before.

• Add a new indexed type Method. This is used to include R6Class methods in workspace symbols but exclusing them from completions.

• Indexers now push symbols onto a list instead of returning a single symbol to their callers. This allows a single indexer to handle multiple symbols.

• The assignment handler now pushes R6Class method symbols in addition to the assigned object.

QA Notes

Add the following to a file:

class <- R6::R6Class(
    'class',
    public = list(
      initialize = function() 'initialize',
      foo = function() {
        1
      }
    ),
    private = list(
      bar = function() {
        2
      },
      not_indexed1 = NULL
    ),
    other = list(
      not_indexed2 = function() {
        3
      }
    )
  )

These symbols should now be available as Workspace symbols (# prefix in command palette): initialize(), foo(), bar() and you should be able to jump straight to the definition of these methods from any file.

The symbols starting with not_indexed should not be available.

All this is tested on the backend side.