Use single best completion item for commands, options, and global options

[baronfel]

This is an attempt to make completions less noisy by using the most relevant available alias for each Subcommand or Option provided for completions. Currently, each of these adds all of its aliases to the pool of available completions, which is quite noisy and can be distracting to a user. Imagine a shell that displays descriptions for completions. Would you want completions at the top-level of the command to emit all 5 Help Option aliases on new rows, along with the same description? I don't think so.

Instead, we try to find the best alias for the Subcommand or Option. What 'best' means varies by context:

• when the user hasn't typed any part of the subcommand or option, the best should be the longest alias. This is typically a most-descriptive form of the option. While such an alias might be a pain to type out manually, with tab completion we can trade precision for verbosity in a way that costs very little effort.
• when the user has typed a part of the subcommand or option, we use that stem to:
  • filter the aliases by those that contain the stem
  • sort those descending by Levenstein distance ---------------------- These two steps are taken from the TypoCorrection middleware, it seemed good and relevant to reuse this algorithm
  • then sort descending by the length of the matching starting stem _/
  • then take the first match

Left to do:

• get feedback on algorithm
• add tests specifically around verifying that only one alias is returned per option

[baronfel]

use alias.ContainsCaseInsensitive extension method instead here to remove TFM split

[KalleOlaviNiemitalo]

the best should be the longest alias

Consider using the first alias, or even Symbol.Name. This would give the app developer better control on which alias is preferred.