[Proposal] Add Autocomplete Provider

[TheBrambleShark]

Though not useful on all platforms (e.g. parsing args in console apps), many systems which utilize Remora.Commands may find an autocomplete or type ahead adapter useful.

On the Remora side, this includes Remora.Discord, which expands Remora.Commands with this functionality.

API Proposal

For this first version, I propose copying several types from Remora.Discord.Commands into Remora.Commands, removing Discord-specific implementation details as necessary. These types include:

• IAutocompleteProvider
• EnumAutocompleteProvider
• Many of the non-Discord-Specific Attributes
• IOperationContext for adding system-specific contextual information.
• InvalidCommandParameterException
• InvalidNodeException
• UnsupportedFeatureException -- Exactly as it says on the tin - failure to create a slash command related to some feature supported by Remora.Commands but not by the backing service (could be Discord, could be anything).
• UnsupportedParameterFeatureException
• Any relevant Extensions
• Likely many of the Services - SimplePrefixMatcher at a bare minimum (99% of consumers will likely have a text box for users to type into where commands are denoted by a prefix.).