Make INI syntax register as handler of .inf files

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.

CHANGELOG.md

@@ -28,6 +28,7 @@
 - Associate `/var/spool/mail/*` and `/var/mail/*` with the `Email` syntax. See #2156 (@cyqsimon)
 - Added cmd-help syntax to scope --help messages. See #2148 (@victor-gp)
 - Slightly adjust Zig syntax. See #2136 (@Enselic)
+- Associate `.inf` files with the `INI` syntax. See #2190 (@Enselic)
 
 ## Themes