[codegen ignore] normalize path separator for Windows, add *.ext tests

[jimschubert]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Add missing tests for simple globbed ignore pattern which is not nested and not explicitly rooted...

See comment in Rule.java:

// case 1
//: If the pattern does not contain a slash '/', Git treats it as
//: a shell glob pattern and checks for a match against the
//: pathname relative to the location of the `.gitignore` file
//: (relative to the toplevel of the work tree if not from a
//: `.gitignore` file).

see #4452