Inefficient regex in `extract_full_summary_from_signature`

As pointed out by https://gist.github.com/prodigysml/d07cd482214c80bfb6d3240454d2f679, this regex (introduced by https://github.com/microsoft/knack/commit/430c39e657d8a424ef9b631782fe0e62a6bed203, #8) is inefficient:

https://github.com/microsoft/knack/blob/e0c14114aea5e4416c70a77623e403773aba73a8/knack/introspection.py#L18

It tries to match a string such as:

        :param command_loader: The command loader that commands will be registered into

As shown in https://regex101.com/, a simple :param r requires 1214 steps to fail.

:param r causes catastrophic backtracking:

This is because \s+, .+? and \s* all match consecutive spaces, thus can trigger many backtrackings.

A better solution is to replace .+? with \w+ to match the parameter name so that backtrackings can be greatly reduced:

\s*(:param)\s+(\w+)\s*:(.*)

Start in knack/introspection.py at the extract_full_summary_from_signature regex around line 18. Reproduce the long-space and malformed inputs shown in the issue, then verify that parameter documentation still extracts correctly while the pathological input no longer causes excessive backtracking. The proposed regex and examples define the expected behavior.