fix(babel): emit forward slashes in generated import paths on Windows

[KallinikosMil]

Motivation

scripts/generate-mappings.ts builds each mapping's path with path.relative, which returns platform-native separators. On Windows that writes backslashes into lib/mappings.json:

"Palette": { "path": "lib\module\theme\tokens", "name": "Palette" }

The babel plugin then interpolates that value straight into an import specifier (`${name}/${mapping.path}` in src/babel/index.js), so the rewritten imports come out as:

import { Palette } from "react-native-paper/lib\module\theme\tokens";

Import specifiers always use forward slashes regardless of platform, so these paths don't resolve — the plugin's output is broken for anyone generating the mappings on Windows.

This normalises the separator to / at the point where the path is produced. index is taken from package.json's main, which is already POSIX, so only the relative() result needed fixing.

Related issue

No existing issue — I ran into it while running the test suite on Windows, where src/babel/__tests__/index.js fails on main for this reason.

Test plan

yarn jest src/babel fails on main on Windows and passes with this change:

- import { Palette } from "react-native-paper/lib/module/theme/tokens";
+ import { Palette } from "react-native-paper/lib\module\theme\tokens";

The regenerated lib/mappings.json now contains "lib/module/theme/tokens" on Windows, matching what the existing __fixtures__/generate-mappings/output.js snapshot expects.

The change is a no-op on Linux/macOS, where path.sep is already /, so the existing CI result is unaffected. Full suite locally: 55 suites / 735 tests passing, lint and typecheck clean.