`extract_full_summary_from_signature` uses undocumented `Match.regs`

[jiasli]

extract_full_summary_from_signature (added by https://github.com/microsoft/knack/commit/430c39e657d8a424ef9b631782fe0e62a6bed203) uses undocumented Match.regs:

https://github.com/microsoft/knack/blob/e0c14114aea5e4416c70a77623e403773aba73a8/knack/introspection.py#L22

According to

• https://stackoverflow.com/questions/27537007/alternative-to-regs-attribute-of-match-object-in-re-in-python3
• https://github.com/python/cpython/issues/62243

it should be replaced by Match.span.

Related: #281