fix(regression): Webpack not working on Windows with new export registry

[dsevillamartin]

Caused by #3842.

Changes proposed in this pull request:

• Use path.relative for module path
• Replace system path separators with /

Initially, since .replace() was hardcoding /, the entire absolute path was being added to flarum.reg.add().
Then, building still errored because it was using \ as path separators, and the following characters weren't valid escape sequences.

ERROR in ./src/common/utils/styleSelectedText.ts 354:32
Module parse failed: Bad character escape sequence (354:32)
File was processed with these loaders:
 * ../../../js-packages/webpack-config/src/autoExportLoader.cjs
 * ../../../node_modules/babel-loader/lib/index.js
You may need an additional loader to handle the result of these loaders.
|   };
| }
> flarum.reg.add('core', 'common\utils\styleSelectedText', styleSelectedText)

Tried to use path.posix, but it can't normalize win32 paths into unix. There's a package for that (slash), but it basically just replaces \ with / - I don't think this implementation should cause any issues.

QA
Build JS. Dist remains the exact same.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[SychO9]

Thanks!