[vcpkg] Add tab completion for Fish

[Link1J]

Adds tab completion for Fish.
Fish is shell, like bash, but is not fully POSIX compliant.

[BillyONeal]

1. I'm not sure if we should offer autocomplete for x- options and commands since those aren't intended to be contractual
2. Since this would be a community supported thing, should we name the whole thing vcpkg integrate install x-fish or something like that?

[Link1J]

Here is my opinion on the points laid out.

1. Removing the x- options makes sense, and will likely be something I will do.
2. a) I don't think using x- for community supported items makes sense, but something like c- would.
3. b) The install part should be used for all of the tab-completes, if it is added. I don't think it should.

[BillyONeal]

Here is my opinion on the points laid out.

1. Removing the x- options makes sense, and will likely be something I will do.

Thanks!

2. a) I don't think using x- for community supported items makes sense, but something like c- would.

Sorry, I should clarify: x- is a marker that the command's behavior might change in the future without warning, and we want the community to have the power to so change features they contribute. I don't think we would want a different prefix there.

3. b) The install part should be used for all of the tab-completes, if it is added. I don't think it should.

Ah, you're right, I messed that up. Pretend I said vcpkg integrate x-fish :)

[Link1J]

Oh, I misunderstood what x- meant.
I have made the changes.

[BillyONeal]

@strega-nil @ras0219 and I met today and confirmed that we like doing this feature; dropping requires:discussion

[strega-nil]

Approving as a simple, experimental fish support :)

[BillyONeal]

Thanks for your contribution!