Fix yarn build-types on Windows

[YevheniiKotyrlo]

Summary:

yarn build-types fails on Windows. Three places in scripts/js-api/build-types treat a native file path as /-separated:

• replaceDefaultExportName takes the module name from filePath.split('/').pop(). A Windows path has no /, so the default export is renamed to $$C:\…\ActionSheetIOS\ActionSheetIOS, and the snapshot step cannot parse the generated .d.ts (Expecting Unicode escape sequence \uXXXX).
• resolveCyclicImportsInDefinition takes the source directory the same way, and writes the rewritten import specifier with path.join, which uses \ on Windows.
• buildApiSnapshot pastes the temp directory into api-extractor.json as it is, so JSON reads its backslashes as escapes and api-extractor gets a projectFolder that does not exist.

This uses path.basename and path.dirname, and writes the paths that end up in an import specifier or in the config with /. On Linux and macOS the output is unchanged.

Changelog:

[INTERNAL] [FIXED] - Fix yarn build-types on Windows

Test Plan:

• Windows 11, Node 26: on main, yarn build-types stops at the snapshot step with the parse error above. With this change it completes, packages/react-native/ReactNativeApi.d.ts comes out identical to the committed snapshot, and yarn build-types --validate passes.
• replaceDefaultExportName-test now builds its path with path.join, so it runs with the platform's separator. On Windows it and the eight resolveCyclicImportsInDefinition cases fail on main and pass with this change.
• jest scripts/js-api passes (161 tests) on Windows and on Linux (WSL2, Node 22.13.0), and yarn build-types --validate passes on Linux. yarn flow-check is clean, and so are eslint and prettier on the changed files.