Feature: Generalize ellipsis as a `..rest` argument (when `rest-args` enabled)

[dgkf]

Closes #133; Inspired by the discussion in #133

vanilla

f <- fn(...) list(...)
f(1, 2, 3)
# list(1, 2, 3)

f <- fn(...) .
f(1, 2, 3)
# Error: object '.' not found

rest-args

f <- fn(...) list(...)
f(1, 2, 3)
# list(1, 2, 3)

f <- fn(...) .
f(1, 2, 3)
# list(1, 2, 3)

• Treats ... as a "rest" argument, similar to any other "rest" argument using the "more" operator (..). When the rest-args experiment is enabled, ... gets parsed as a named ellipsis argument with the name ..

Other enhancements

• Adds parse, and parse_and_eval as methods of CallStack to make it easier to write tests that make use of experiments
• Cleans up some legacy uses of session.local.parse_input_with and replaces with parser_config.parse_input
• Fixes feature disabled error message to display correct command to enabled features.
• CallStack::default() now defaults to a global environment with pre-populated builtin functions instead of an empty environment.
• Renamed parse_pairlist to parse_list_elements since we don't use a lisp-style "pairlist"