Add basic handling for nested lists

[RohanM]

https://app.bugsnag.com/the-conversation/tc/errors/6576cc832f734a0008ef8789?filters[error.status]=open&filters[event.since]=30d

We've seen a slow but continual stream of errors related to markdown rendering of nested lists. We'd like to offer at least a basic solution to this common use case. The output of this change isn't valid markdown, but represents a human-
readable result, which is at least a step forward.

Before:

NoMethodError: undefined method `strip' for ["* My list item"]

After:

* * My nested list item

1. 1. My nested list item

[RohanM]

[upmark] Add basic nested list handling

[tiagoamaro]

Checking out these new tests to the main branch will raise the following errors in the test suite, as expected:

╰─$ bundle exec rspec                                              

Randomized with seed 12685
..F................F..........................................................................

Failures:

  1) Upmark.convert nested unordered lists generates readable output
     Failure/Error: children = element[:children].map {|value| value.strip != "" ? value : nil }.compact
     
     NoMethodError:
       undefined method `strip' for ["* List item\n"]:Array
     # ./lib/upmark/transform/markdown.rb:51:in `block (2 levels) in <class:Markdown>'
     # ./lib/upmark/transform/markdown.rb:51:in `map'
     # ./lib/upmark/transform/markdown.rb:51:in `block in <class:Markdown>'
     # ./lib/upmark/transform_helpers.rb:22:in `block (2 levels) in element'
     # ./lib/upmark.rb:20:in `convert'
     # ./spec/acceptance/upmark_spec.rb:8:in `actual'
     # ./spec/acceptance/upmark_spec.rb:4:in `block (3 levels) in <top (required)>'
     # ./spec/acceptance/upmark_spec.rb:357:in `block (3 levels) in <top (required)>'

  2) Upmark.convert nested ordered lists generates readable output
     Failure/Error: children = element[:children].map {|value| value.strip != "" ? value : nil }.compact
     
     NoMethodError:
       undefined method `strip' for ["1. List item\n"]:Array
     # ./lib/upmark/transform/markdown.rb:56:in `block (2 levels) in <class:Markdown>'
     # ./lib/upmark/transform/markdown.rb:56:in `map'
     # ./lib/upmark/transform/markdown.rb:56:in `block in <class:Markdown>'
     # ./lib/upmark/transform_helpers.rb:22:in `block (2 levels) in element'
     # ./lib/upmark.rb:20:in `convert'
     # ./spec/acceptance/upmark_spec.rb:8:in `actual'
     # ./spec/acceptance/upmark_spec.rb:4:in `block (3 levels) in <top (required)>'
     # ./spec/acceptance/upmark_spec.rb:373:in `block (3 levels) in <top (required)>'

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94 examples, 2 failures