Good feature.

It's not very usual to put a function definition in a C header file (although quite normal to put functions inside templates inside c++ headers I think...). Would it be possible to make this a C file, mark the function as non-static, and add a predeclaration to the top of scanner.c?

yeah, that sounds sensible. No idea how to tell tree-sitter about this file though when running tree-sitter generate. It doesn't link it on its own at least.
Since we're only using the header in one C file, does it make much of a difference to use a header? I'd assume that an extra C file would have to be specified explicitly in downstream apps like nvim-treesitter.

tree-sitter/tree-sitter#1262

seems we're out of luck

At least the variant with the header appears to install fine in Neovim!

Okay, well the header sounds like an okay solution :)

Shame we can't have multiple source files though...

yeah well, if that issue I linked is going to be implemented we can just change it :) so I'll go ahead and merge.