Add macos-latest to CI matrix

[purcell]

This should provide coverage of ARM-based MacOS, e.g. for issues like #327.

[lfittl]

This should provide coverage of ARM-based MacOS, e.g. for issues like #327.

Good idea, but looks like that succeeded despite the reported failure on 15.4. I'm assuming that the GH action runners are not updated yet, making this less useful. I'll have to check if there is any other downside to enabling this (I recall macOS runners had lower limits?) but if there are no downsides happy to enable it for extra coverage.

[lfittl]

I ended up adding this on the libpg_query side, where I think this matters most: pganalyze/libpg_query@e46bdf8

Unless you think there is a chance there will be macOS and Ruby-specific issues, I'd prefer we don't add more test runs here (since Linux and macOS Ruby builds are sufficiently similar).