Implement substitute() effect handler

[lionel-]

Progress towards #1338

Adds a Quote handler for substitute() calls.

In the following example, goto-def, find-refs, and rename should:

• See through symbols defined in the current environment (arg and obj).
• Treat all other symbols as quoted/inert, including those defined in a parent (foo, foobaz).

foo <- function() {}
foobaz <- 1

function(arg) {
  obj <- 1
  substitute(foo(arg, obj, foobaz))
}

Positron Release Notes

New Features

• Symbol navigation now works inside substitute() calls. Quoted parts remain inert, whereas substituted parts now goto-definition, find-references, or rename.

Bug Fixes

• N/A