[globset] supporting paths with Windows separators

#### Describe your feature request

Hi Andrew! Thanks for all your wonderful crates.

I noticed that `globset` currently seems to have certain Unix assumptions baked in, such as `literal_separator` only excluding `/` and not `\`.

I'm wondering if it would be possible to run it in a mode where it supports Windows paths, without having the path converted from `\` to `/`. The general approach I'm thinking is:
* globs are still provided in Unix format with `/` path separators.
* `literal_separator` is turned on.
* a new config `match_backslash_separator` is provided (people can turn it on on `#[cfg(windows)]`, or maybe it could take an enum with values like `Never`, `OnWindows`, `Always`).
* internally, `globset` converts `/` in the glob to `[/\\]` in the regex, and makes it so that `literal_separator` doesn't match either `/` or `\`.

I might have missed something but I believe this should work. (It could also run in a "canonical Windows path" mode, where `/` in the glob *only* matches `\` in the string, but I'm not sure if there's a good use case for that.)

Start by reading globset's handling of literal_separator and its glob-to-regex conversion. Evaluate the proposed match_backslash_separator configuration and verify that slash separators and literal matching behave correctly for Windows paths without converting the input path.

Written by the indexing model from the issue text.