We need to type `inf` and `INF` as strings in `INI.sublime-syntax`,
otherwise `yaml-rust` interprets them as real numbers ("infinity") and
they do not get registered as file extensions:
/Users/martin/src/yaml-rust # https://github.com/chyh1990/yaml-rust
% cargo run --example dump_yaml ~/src/bat/assets/syntaxes/02_Extra/INI.sublime-syntax
---
String("name"):
String("INI")
String("file_extensions"):
String("ini")
String("INI")
Real("inf")
Real("INF")
...
Also add a regression test.
