Thank you for doing this, @jtbandes! I'd been looking at the sorry situation of Swift syntax highlighting in VS Code just recently, coincidentally, and was really rather depressed about it - by all appearances it was abandoned years ago.
My interest is not actually in VS Code itself, but rather that these grammar files are also used in things like WordPress plug-ins (e.g. Code Block Pro). It's a real struggle to find any syntax highlighting plug-in for WordPress that does even just a half-decent job with Swift.
The other problem with Swift syntax highlighting is poor theme support, even in VS Code surprisingly - I thought there was a non-trivial number of people using Swift in VS Code, but judging from the abysmal support for Swift in all the popular themes (and even supposedly purpose-built-for-Swift themes), that is not the case. Do you have any plans to tackle that as well, perchance?
(I spent several long days trying to build a new theme for VS Code specifically for Swift, and while it didn't take long at all to out-do every existing theme, I realised it'll take way more time to actually complete it… writing good VS Code themes is extremely laborious 
)