Add support for setting the cursor visibility in Terminal

[zadjii-msft]

Adds support for setting the cursor visibility in Terminal. Visibility
is a property entirely independent from whether the cursor is "on" or
not. The cursor blinker should change the "IsOn" property. It was
actually changing the "Visible" property, which was incorrect. This PR
additionally corrects the naming of the method used by the cursor
blinker, and makes it do the right thing.

I added a pair of tests, one taken straight from conhost. In
copy-pasting that test, I took it a step further and implemented
^[[?12h, ^[[?12l, which enables/disables cursor blinking, for the
TerminalCore. THIS DOES NOT ADD SUPPORT FOR DISABLING BLINKING IN THE
APP. Conpty doesn't emit the blinking on/off sequences quite yet, but
when it does, the Terminal will be ready.

References

• I'd bet this conflicts with Add support for solid cursor blinking style #2892
• This isn't a solution for Add support for turning off cursor blink #1379
• There shockingly isn't an issue for cursor blink state via conpty...?

PR Checklist

• Closes Terminal: add support for showing/hiding the cursor (DECSET/DECRST 25, DECTCEM) #3093
• Closes Double cursors in neovim terminal buffer (and fzf, probably more) #3499
• Closes Cursor position seems to be mispositioned when using WLS2 & git rebase #4644
• I work here
• Tests added/passed
• [n/a] Requires documentation to be updated

[DHowett-MSFT]

This seems totally sane to me.

[DHowett-MSFT]

independently

[zadjii-msft]

asfdgkajenfklaerba classic me

[miniksa]

Man, go read that spell check PR #4799 so we can make a computer catch these things for you.

[miniksa]

Yeah, this is fine. I don't remember what all the flags on Cursor do or are supposed to do anymore. One day we should probably reconcile all of them because I bet there's like 4-6 of them that are effectively duplicates with each other but just make the logic harder to follow.

Anyway... as long as this improves things for now, I'm fine with it. I don't see anything obviously wrong.

[DHowett-MSFT]

@msftbot merge this in 1 minute

[ghost]

Hello @DHowett-MSFT!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Fri, 13 Mar 2020 17:39:14 GMT, which is in 1 minute

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".