feat: add support for providing serialized answers to multiselect choice questions

[sisp]

I've added support for providing serialized answers to multiselect choice questions.

This feature enables two use cases:

1. The default value can be templated by rendering default choice items as a YAML-style list.
2. An answer can be passed via the CLI flag --data as a YAML-style list. Resolves #1474.

The answer to a multiselect choice question is a list of choice items. While it seems that the most obvious solution is to parse the answer using a YAML parser, I think it's not the right approach because the list items should be parsed according to type: <type> and not necessarily using a YAML parser, and it's inconsistent with other question types to first parse them as YAML and afterwards parse according to type: <type>. But I've found a way to shallowly parse the serialized answer as a YAML-style list without parsing the items. This way, the items remain raw strings, delegating their parsing to the parser according to type: <type>. There is one subtlety: YAML strings may be quoted or (sometimes) unquoted. To avoid double-quoting after the shallow list parsing, the quotes of a YAML string are stripped.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 97.89%. Comparing base (7f4a1f8) to head (f2ff775). :warning: Report is 107 commits behind head on master.

Files with missing lines	Patch %	Lines
copier/_user_data.py	92.85%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2189      +/-   ##
==========================================
- Coverage   98.03%   97.89%   -0.14%     
==========================================
  Files          55       55              
  Lines        6005     6038      +33     
==========================================
+ Hits         5887     5911      +24     
- Misses        118      127       +9     

Flag	Coverage Δ
unittests	97.89% <96.15%> (-0.14%)	:arrow_down:

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