Fix prompt parser default step transformer w/ test

from typing import List

import lark

try:

    from modules.shared import opts

except:

    pass

# a prompt like this: "fantasy landscape with a [mountain:lake:0.25] and [an oak:a christmas tree:0.75][ in foreground::0.6][ in background:0.25] [shoddy:masterful:0.5]"

# will be represented with prompt_schedule like this (assuming steps=100):

# [25, 'fantasy landscape with a mountain and an oak in foreground shoddy']

    [[5, 'a  c'], [10, 'a {b|d{ c']]

    >>> g("((a][:b:c [d:3]")

    [[3, '((a][:b:c '], [10, '((a][:b:c d']]

    >>> g("[a|(b:1.1)]")

    [[1, 'a'], [2, '(b:1.1)'], [3, 'a'], [4, '(b:1.1)'], [5, 'a'], [6, '(b:1.1)'], [7, 'a'], [8, '(b:1.1)'], [9, 'a'], [10, '(b:1.1)']]

    """

    def collect_steps(steps, tree):

                yield args[0].value

            def __default__(self, data, children, meta):

                for child in children:

                    try:

                        if opts.use_old_prompt_parser_default_step_transformer:

                            yield from child

                        else:

                            yield child

                    except:

                        yield child

        return AtStep().transform(tree)

    def get_schedule(prompt):