[settings-view] Fix Package keymap compatibility check

[confused-Techie]

It turns out that for quite some time the way the package's keymap section checks for platform compatibility has been broken.

I've detailed the reason why in #1160, but basically, if a keymap listed multiple platforms, such as linux and win32 the compatibility check would just see that it works on a platform other than the current one, and would think it's incompatible and not show it to the user.

So I've gone ahead and fixed this check much in the same way we already check in settings-view/lib/keybindings-panel.js except I've expanded out on this functionality a bit, by creating a method for this check.

Also, I've given the ability to pass in a platform rather than going with the current one, which makes it much easier to test this, and means if we wanted to in the future we could purposefully grab incompatible keymaps.

And since it's not testable, of course I've written new tests to be able to test this behavior.

Resolves #1160

[savetheclocktower]

I haven't tried to manually verify this — the xml-formatter package that triggered this fix always did show keymaps on my platform — but the code seems solid and the unit tests are appreciated.

Assuming that this has been verified to fix the original issue, 👍 .

[confused-Techie]

Thanks for the review!
It hasn't been manually tested, as I was having difficulty testing getting errors from Windows about the app being incompatible.

But I'll go ahead and download the generated bins to give it a quick manual test

[confused-Techie]

Alright, I can confirm that this does in fact solve the issue for xml-formatter and others like it.
Merging now!