Support clink-like lua completion scripts

[remkop]

Clink offers the ability to customize its shell completion with lua scripts. Some of clink's own functionality, like built-in completions, are also implemented in lua. This integration is very powerful; it makes it possible to customize the built-in completion as well as add custom completion logic.

In addition to the abilities of the lua programming language, Clink offers a parsing API that greatly simplifies the development of custom completion scripts.

This is related to our discussion in #41: supporting lua and Clink's argument parser API is one way to address the concern about the complexity of completion scripts for non-trivial commands like git.

Another benefit I can see is that by supporting this API, existing lua completion scripts can be used as is in yori, which may make the idea of migrating to yori much more attractive for clink users.
Published collections of clink completion scripts like https://github.com/vladimir-kotikov/clink-completions would instantly add to yori's power and hopefully foster community enthusiasm and engagement, since the clink project itself appears to be abandoned.

TBD:

• The clink parser API does not have support for descriptions, so it may need to be extended to implement feature requests like Completion candidate descriptions #42
• Clink functionality can be accessed in lua completion scripts via the clink variable. I imagine that you would want to expose a yori variable instead (but expose clink as an alias for compatibility with clink scripts)