Test symbols rather than strings for OS detection

[geoffp]

This fixes #27 for me. Just tests for symbols rather than strings.

[orzechowskid]

thanks for this! I don't have a Mac to test with so I've been depending on others to help out 😬

it looks like github actions are failing due to unrelated issues (nodejs/node-gyp#2869 , if I'm reading the logs correctly) so it might be a bit before I can get an official release out but this change looks good. thanks again!

[geoffp]

@orzechowskid Happy to help! I suuuper want this to work, and I'm happy to help however I can. I have Mac OS and FreeBSD on my desktop right now, so I can test stuff on weird OSes; feel free to reach out if you need something.

I actually still have not been able to get highlighting to work in a tsx-ts-mode or typescript-ts-mode buffer, despite this fix, so the journey continues. But that's another issue! Any tips on debugging process are greatly appreciated. I'm still an elisp novice.